Imaging device including at least one unit pixel cell

ABSTRACT

An imaging device includes at least one unit pixel cell including a photoelectric converter that converts incident light into electric charges. The photoelectric converter includes: a first electrode; a light-transmitting second electrode; a first photoelectric conversion layer disposed between the first electrode and the second electrode and containing a first material having an absorption peak at a first wavelength; and a second photoelectric conversion layer disposed between the first photoelectric conversion layer and the second electrode and containing a second material having an absorption peak at a second wavelength different from the first wavelength. The absolute value of the ionization potential of the first material is larger by at least 0.2 eV than the absolute value of the ionization potential of the second material.

BACKGROUND 1. Technical Field

The present disclosure relates to an imaging device.

2. Description of the Related Art

There is a need for an imaging device having high sensitivity not onlyin the visible range but also in the longer wavelength near infrared andinfrared ranges for applications to security cameras and vehicle-mountedcameras for supporting safe driving. In MOS (metal oxide semiconductor)imaging devices that have been widely used, signals stored in PNphotodiodes forming pixels are read through amplifier circuits includingMOS field-effect transistors (MOSFETs). Silicon is a semiconductormaterial widely used for MOS imaging devices but cannot absorb muchlight with a wavelength of about 1,100 nm or longer because of itsphysical property limitations. It is therefore difficult for an imagesensor using a silicon substrate to have sensitivity to long wavelengthlight. It is also known that, in the image sensor using the siliconsubstrate, the sensitivity of the sensor to light in the near infraredrange with a wavelength of 800 nm or longer is lower than thesensitivity to light in the visible range because of the wavelengthdependence of the optical absorption coefficient of silicon.

For example, Japanese Unexamined Patent Application Publication No.2008-227091 and U.S. Patent Application Publication No. 2014/0001455propose a technique in which a photoelectric convertor that uses anorganic material as a photoelectric conversion material and detectsinfrared light and a photoelectric convertor that detects visible lightare stacked in a vertical direction. Generally, an imaging device usingan organic material has a specific absorption spectrum originating fromthe skeleton of the organic material, i.e., a photoelectric conversionmaterial. Therefore, although silicon has broad spectral sensitivityover a wide wavelength range, it is difficult for this imaging device tohave spectral sensitivity over a wide wavelength range. In view of this,Japanese Patent No. 4511441 proposes a technique in which, in RGB colorimaging, voltages are applied to pixels individually in order to obtainuniform spectral sensitivity in the target wavelength range.

SUMMARY

In one general aspect, the techniques disclosed here feature an imagingdevice including at least one unit pixel cell including a photoelectricconverter that converts incident light into electric charges. Thephotoelectric converter includes a first electrode, a second electrodeconfigured to transmit the incident light, a first photoelectricconversion layer disposed between the first electrode and the secondelectrode and containing a first material having an absorption peak at afirst wavelength, and a second photoelectric conversion layer disposedbetween the first photoelectric conversion layer and the secondelectrode and containing a second material having an absorption peak ata second wavelength different from the first wavelength. The absolutevalue of the ionization potential of the first material is larger by atleast 0.2 eV than the absolute value of the ionization potential of thesecond material.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an exemplary circuit structure ofan imaging device according to an embodiment of the present disclosure;

FIG. 2 is a schematic cross-sectional view showing an exemplary devicestructure of a unit pixel cell in the imaging device according to theembodiment of the present disclosure;

FIG. 3 is a schematic cross-sectional view showing an example of aphotoelectric converter;

FIG. 4A is an illustration showing an example of a material usable for aphotoelectric conversion layer;

FIG. 4B is an illustration showing another example of the materialusable for the photoelectric conversion layer;

FIG. 4C is an illustration showing another example of the materialusable for the photoelectric conversion layer;

FIG. 5 is a schematic cross-sectional view showing another example ofthe photoelectric converter;

FIG. 6 is an energy diagram in a still another example of thephotoelectric converter;

FIG. 7 is an illustration showing the structural formula of CZBDF;

FIG. 8 is an energy diagram of a photoelectric conversion structure inwhich the positions of a first photoelectric conversion layer and asecond photoelectric conversion layer in the photoelectric conversionstructure shown in FIG. 6 are exchanged with each other;

FIG. 9 is a schematic cross-sectional view showing a cross section oftwo adjacent unit pixel cells in a photosensitive region;

FIG. 10 is a schematic plan view showing an example of the arrangementof optical filters;

FIG. 11 is a schematic plan view showing another example of thearrangement of the optical filters;

FIG. 12 is a schematic diagram showing another example of the circuitstructure of the imaging device according to the embodiment of thepresent disclosure;

FIG. 13 is a schematic cross-sectional view showing another example ofthe arrangement of the optical filters;

FIG. 14 is a graph showing the voltage dependence of the externalquantum efficiency of a sample in Example 1-1;

FIG. 15 is a graph showing the relation between an applied electricfield and the external quantum efficiency of the sample in Example 1-1at wavelengths of 460 nm, 540 nm, 680 nm, and 880 nm;

FIG. 16 is a graph showing the voltage dependence of the externalquantum efficiency of a sample in Example 1-2;

FIG. 17 is a graph showing the voltage dependence of the externalquantum efficiency of a sample in Example 1-3;

FIG. 18 is a graph showing the voltage dependence of the externalquantum efficiency of a sample in Example 2-1;

FIG. 19 is a graph showing the voltage dependence of the externalquantum efficiency of a sample in Comparative Example 1;

FIG. 20 is an energy diagram of a sample in Example 2-2;

FIG. 21 is a graph showing the voltage dependence of the externalquantum efficiency of the sample in Example 2-2;

FIG. 22 is an energy diagram of a sample in Comparative Example 2; and

FIG. 23 is a graph showing the voltage dependence of the externalquantum efficiency of the sample in Comparative Example 2.

DETAILED DESCRIPTION

In security cameras and vehicle-mounted cameras, images suitable for thepurpose of monitoring or driving support may be obtained when the imagesare captured using visible light during the daytime and using infraredlight during the nighttime. There is therefore a need for an imagingdevice with a switchable image acquirable wavelength band or switchablespectral sensitivity characteristics.

One non-limiting and exemplary embodiment provides an imaging devicewith a switchable image acquirable wavelength band or switchablespectral sensitivity characteristics.

Aspects of the present disclosure will be summarized below.

[Item 1] An imaging device according to Item 1 of the present disclosureincludes

at least one unit pixel cell including a photoelectric converter thatconverts incident light into electric charges.

The photoelectric converter includes

a first electrode,

a second electrode configured to transmit the incident light,

a first photoelectric conversion layer disposed between the firstelectrode and the second electrode and containing a first materialhaving an absorption peak at a first wavelength, and

a second photoelectric conversion layer disposed between the firstphotoelectric conversion layer and the second electrode and containing asecond material having an absorption peak at a second wavelengthdifferent from the first wavelength.

An absolute value of an ionization potential of the first material islarger by at least 0.2 eV than an absolute value of an ionizationpotential of the second material.

In this structure, by changing the voltage applied between the first andsecond electrodes, the spectral sensitivity characteristics of thephotoelectric converter can be changed electrically.

[Item 2] In the imaging device according to Item 1,

one of the first and second wavelengths may fall within a visiblewavelength range, and

the other of the first and second wavelengths may fall within aninfrared wavelength range.

The visible wavelength range is a wavelength range of, for example, 380nm or more and less than 750 nm, and the infrared wavelength range is awavelength range of 750 nm or more.

[Item 3] In the imaging device according to Item 1,

the first wavelength may fall within a visible wavelength range, and

the second wavelength may fall within an infrared wavelength range.

In this structure, the sensitivity of the imaging device in the infraredwavelength range can be electrically changed.

[Item 4] In the imaging device according to any of Items 1 to 3,

the first material may contain electron-donating molecules, and thesecond material may contain electron-donating molecules.

[Item 5] In the imaging device according to any of Items 1 to 4,

the first photoelectric conversion layer may further containelectron-accepting molecules, and the second photoelectric conversionlayer may further contain electron-accepting molecules.

[Item 6] The imaging device according to any of Items 1 to 5 may furtherinclude

a voltage application circuit electrically connected to the secondelectrode,

wherein the voltage application circuit may selectively apply a firstvoltage or a second voltage different from the first voltage between thefirst electrode and the second electrode.

In this structure, one voltage selected from the plurality of voltagescan be applied between the first electrode and the second electrodeaccording to the polarity of the electric charges collected by the firstelectrode.

[Item 7] In the imaging device according to Item 6,

an absolute value of the second voltage may be larger than an absolutevalue of the first voltage,

an external quantum efficiency of the photoelectric converter at thesecond wavelength corresponding to the absorption peak of the secondmaterial when the second voltage is applied between the first electrodeand the second electrode may be larger than an external quantumefficiency of the photoelectric converter at the second wavelength whenthe first voltage is applied between the first electrode and the secondelectrode, and

a difference between the external quantum efficiency of thephotoelectric converter at the second wavelength when the second voltageis applied and the external quantum efficiency of the photoelectricconverter at the second wavelength when the first voltage is applied maybe larger than a difference between an external quantum efficiency ofthe photoelectric converter at the first wavelength corresponding to theabsorption peak of the first material when the second voltage is appliedand an external quantum efficiency of the photoelectric converter at thefirst wavelength when the first voltage is applied.

[Item 8] In the imaging device according to Item 7,

the external quantum efficiency of the photoelectric converter at thesecond wavelength when the second voltage is applied may be at leasttwice the external quantum efficiency of the photoelectric converter atthe second wavelength when the first voltage is applied.

[Item 9] In the imaging device according to any of Items 1 to 8,

the photoelectric converter may further include a mixture layercontaining the first material and the second material.

[Item 10] In the imaging device according to any of Items 1 to 9,

the at least one unit pixel cell may include a first unit pixel cell anda second unit pixel cell.

[Item 11] The imaging device according to Item 10 may further include

a color filter facing the second electrode of the first unit pixel cell.

With this structure, by changing the voltage applied between each firstelectrode and a corresponding second electrode, the image obtained canbe switched, for example, between an RGB image and an image using redlight and infrared light, green light and infrared light, and blue lightand infrared light.[Item 12] The imaging device according to Item 11 may further include

an infrared pass filter facing the second electrode of the second unitpixel cell.

With this structure, unit pixel cells that output RGB image signals andalso a unit pixel cell that outputs an image signal using infrared lightcan be present in the photosensitive region.

[Item 13] The imaging device according to Item 12 may further include

an infrared cut filter facing the color filter.

With this structure, a camera that can acquire an RGB color image and animage using infrared light simultaneously can be provided.

[Item 14] In the imaging device according to any of Items 10 to 13,

the second electrode of the first unit pixel cell and the secondelectrode of the second unit pixel cell may be a single continuouselectrode.

[Item 15] In the imaging device according to any of Items 10 to 14,

the first photoelectric conversion layer of the first unit pixel celland the first photoelectric conversion layer of the second unit pixelcell may be a single continuous layer, and

the second photoelectric conversion layer of the first unit pixel celland the second photoelectric conversion layer of the second unit pixelcell may be a single continuous layer.

[Item 16] In the imaging device according to Item 1,

the first wavelength may fall within an infrared wavelength range, and

the second wavelength may fall within a visible wavelength range.

In the present disclosure, all or a part of any of circuit, unit,device, part or portion, or any of functional blocks in the blockdiagrams may be implemented as one or more of electronic circuitsincluding, but not limited to, a semiconductor device, a semiconductorintegrated circuit (IC) or an LSI. The LSI or IC can be integrated intoone chip, or also can be a combination of plural chips. For example,functional blocks other than a memory may be integrated into one chip.The name used here is LSI or IC, but it may also be called system LSI,VLSI (very large scale integration), or ULSI (ultra large scaleintegration) depending on the degree of integration. A FieldProgrammable Gate Array (FPGA) that can be programmed aftermanufacturing an LSI or a reconfigurable logic device that allowsreconfiguration of the connection or setup of circuit cells inside theLSI can be used for the same purpose.

Further, it is also possible that all or a part of the functions oroperations of the circuit, unit, device, part or portion are implementedby executing software. In such a case, the software is recorded on oneor more non-transitory recording media such as a ROM, an optical disk ora hard disk drive, and when the software is executed by a processor, thesoftware causes the processor together with peripheral devices toexecute the functions specified in the software. A system or apparatusmay include such one or more non-transitory recording media on which thesoftware is recorded and a processor together with necessary hardwaredevices such as an interface.

Embodiments of the present disclosure will be described with referenceto the drawings. In examples described in the following embodiments,positive and negative charges (typically hole-electron pairs) aregenerated by photoelectric conversion, and the positive charges (e.g.,holes) are detected as signal charges. The present disclosure is notlimited to the following embodiments. One embodiment can be combinedwith another embodiment. In the following description, the same orsimilar components are denoted by the same reference numerals, andredundant description may be omitted.

(Structure of Imaging Device)

FIG. 1 schematically shows an exemplary circuit structure of an imagingdevice according to an embodiment of the present disclosure. The imagingdevice 101 shown in FIG. 1 includes a plurality of unit pixel cells 14and peripheral circuits.

Four unit pixel cells 14 arranged in a 2×2 matrix are shown in FIG. 1.Specifically, in this example, the plurality of unit pixel cells 14 arearranged two-dimensionally, i.e., in row and column directions, on, forexample, a semiconductor substrate and form a photosensitive region (apixel region). It will be appreciated that the number of unit pixelcells 14 and their arrangement are not limited to those in the exampleshown in FIG. 1. For example, the imaging device 101 may be a linesensor. In this case, a plurality of unit pixel cells 14 are arrangedone-dimensionally. In the present specification, the row direction isthe direction in which the rows extend, and the column direction is thedirection in which the columns extend. Specifically, the verticaldirection in the drawing sheet in FIG. 1 is the column direction, andthe horizontal direction is the row direction. The number of unit pixelcells 14 may be 1.

Each of the unit pixel cells 14 includes a photoelectric converter 10and a charge detection circuit 25 electrically connected to thephotoelectric converter 10. In this example, the photoelectric converter10 includes a pixel electrode 50, a counter electrode 52, and aphotoelectric conversion structure 51 disposed therebetween. The chargedetection circuit 25 includes an amplification transistor 11, a resettransistor 12, and an address transistor 13.

As will be described later in detail, in the present embodiment of thepresent disclosure, the photoelectric conversion structure 51 in thephotoelectric converter 10 includes a layered structure including firstand second photoelectric conversion layers. The first photoelectricconversion layer contains a first material, and the second photoelectricconversion layer contains a second material. In one aspect of thepresent disclosure, the impedance of the first photoelectric conversionlayer is higher than the impedance of the second photoelectricconversion layer. In this structure, by changing a voltage appliedbetween the pixel electrode 50 and the counter electrode 52, thespectral sensitivity characteristics of the photoelectric converter 10can be changed. By changing the spectral sensitivity characteristics ofeach unit pixel cell 14, its image acquirable wavelength band can bechanged. In the present specification, for the sake of simplicity, theterm “impedance” may be used to mean the “absolute value of theimpedance.” The impedance of the first photoelectric conversion layerand the impedance of the second photoelectric conversion layer may beimpedances at a frequency of 1 Hz with the first and secondphotoelectric conversion layers not irradiated with light.

In another aspect of the present disclosure, the absolute value of theionization potential of the first material is larger by at least 0.2 eVthan the absolute value of the ionization potential of the secondmaterial. As will be described later, when the difference in ionizationpotential between the first and second materials is somewhat large, evenif the difference in impedance between the first and secondphotoelectric conversion layers is small, the spectral sensitivitycharacteristics of the photoelectric converter 10 can be changed bychanging the voltage applied between the pixel electrode 50 and thecounter electrode 52.

In the structure exemplified in FIG. 1, the imaging device 101 includesa voltage application circuit 60. For example, the voltage applicationcircuit 60 is connected to a plurality of bias voltage lines 16 providedfor respective rows of unit pixel cells 14. In this example, the counterelectrode 52 of the photoelectric converter 10 of each of the unit pixelcells 14 is connected to a corresponding one of the plurality of biasvoltage lines 16.

The voltage application circuit 60 may be a circuit that can generate atleast two voltages with different absolute values. During operation ofthe imaging device 101, the voltage application circuit 60 supplies, forexample, one of the plurality of different voltages selectively to theunit pixel cells 14. The voltage supplied from the voltage applicationcircuit 60 may be referred to as a switching voltage. For example, thevoltage application circuit 60 supplies one of a first voltage VA and asecond voltage VB larger in absolute value than the first voltage VAselectively to the unit pixel cells 14. The voltage application circuit60 is not limited to a specific power supply circuit and may be acircuit that generates prescribed voltages or a circuit that converts avoltage supplied from another power source into a prescribed voltage.Typically, the voltage application circuit 60 is disposed outside thephotosensitive region as part of the peripheral circuits.

In the present embodiment, by changing the potential of each counterelectrode 52 through a corresponding bias voltage line 16, the voltageapplied between the counter electrode 52 and a corresponding pixelelectrode 50 is changed. By changing the voltage applied between thepixel electrode 50 and the counter electrode 52, the spectralsensitivity characteristics of the photoelectric converter 10 arechanged. When each pixel electrode 50 collects holes as signal charges,it is only necessary that the potential of the pixel electrode 50 belower than the potential of its counter electrode 52.

In each photoelectric conversion structure 51, the pixel electrode 50collects positive or negative charges (e.g., holes) generated byphotoelectric conversion. When holes are used as signal charges,voltages used as the first voltage VA and the second voltage VB are suchthat the potential of the counter electrode 52 is higher than thepotential of the pixel electrode 50. The pixel electrode 50 iselectrically connected to a gate electrode of the amplificationtransistor 11 in the charge detection circuit 25, and the signal charges(holes in this case) collected by the pixel electrode 50 are stored in acharge storage node 24 located between the pixel electrode 50 and thegate electrode of the amplification transistor 11. In the presentembodiment, the signal charges are holes but may be electrons.

When the signal charges are stored in the charge storage node 24, avoltage corresponding to the amount of the signal charges is applied tothe gate electrode of the amplification transistor 11. The amplificationtransistor 11 amplifies the applied voltage and outputs the resultingvoltage as a signal voltage. The output from the amplificationtransistor 11 is read selectively by the address transistor 13. In theexample shown in FIG. 1, the charge detection circuit 25 includes thereset transistor 12. As shown in FIG. 1, one of the source and drainelectrodes of the reset transistor 12 is electrically connected to thepixel electrode 50. By switching the reset transistor 12 on, the signalcharges stored in the charge storage node 24 can be reset. In otherwords, the reset transistor 12 can reset the potential of the gateelectrode of the amplification transistor 11 and the potential of thepixel electrode 50 of the photoelectric converter 10.

As illustrated, power source lines 21, vertical signal lines 17, addresssignal lines 26, and reset signal lines 27 are connected to therespective unit pixel cells 14. The power source lines 21 are connectedto the source or drain electrodes (typically the drain electrodes) ofthe amplification transistors 11 and supply a prescribed power sourcevoltage (e.g., 3.3 V) to the unit pixel cells 14. The vertical signallines 17 are connected to the source or drain electrodes (typically thesource electrodes) of the address transistors 13. The address signallines 26 are connected to the gate electrodes of the address transistors13, and the reset signal lines 27 are connected to the gate electrodesof the reset transistors 12.

In the structure exemplified in FIG. 1, the imaging device 101 includes,in addition to the voltage application circuit 60, peripheral circuitsincluding a vertical scanning circuit 15, a horizontal signal readingcircuit 20, a plurality of column signal processing circuits 19, aplurality of load circuits 18, and a plurality of inverting amplifiers22. The vertical scanning circuit 15 is referred to also as a rowscanning circuit. The horizontal signal reading circuit 20 is referredto also as a column scanning circuit. The column signal processingcircuits 19 are referred to also as column signal storage circuits.

The vertical scanning circuit 15 is connected to the address signallines 26 and the reset signal lines 27, selects any of the rows of unitpixel cells 14, reads signal voltages from the selected unit pixelcells, and resets the potential of each of the pixel electrodes 50. Thevertical signal lines 17 are provided for the respective columns of unitpixel cells 14, and each of the unit pixel cells 14 is connected to acorresponding one of the vertical signal lines 17. As illustrated, theload circuits 18 and the column signal processing circuits 19, as wellas the vertical signal lines 17, are provided for the respective columnsof unit pixel cells 14 and are each electrically connected to at leastone unit pixel cell 14 disposed in a corresponding column through acorresponding vertical signal line 17. The load circuits 18 and theamplification transistors 11 form source follower circuits. The columnsignal processing circuits 19 perform noise suppression signalprocessing typified by correlated double sampling, analog-digitalconversion (A/D conversion), etc. The horizontal signal reading circuit20 is electrically connected to the plurality of column signalprocessing circuits 19. The horizontal signal reading circuit 20sequentially reads signals from the plurality of column signalprocessing circuits 19 and outputs the signals to a horizontal commonsignal line (not shown).

The vertical scanning circuit 15 applies a row selection signal to thegate electrode of each address transistor 13 through its correspondingaddress signal line 26, and the row selection signal controls theaddress transistor 13 to switch it on and off. The row selection signalis applied to a row to be read, and this row is scanned and selected.Signal voltages are read from unit pixel cells 14 in the selected rowthrough the respective vertical signal lines 17. The vertical signallines 17 transmit the signal voltages read from the unit pixel cells 14selected by the vertical scanning circuit 15 to the respective columnsignal processing circuits 19. Moreover, the vertical scanning circuit15 applies a reset signal to the gate electrode of each reset transistor12 through a corresponding reset signal line 27, and the reset signalcontrols the reset transistor 12 to switch it on and off. In thismanner, signal charges in the charge storage node 24 of each unit pixelcell 14 with the reset transistor 12 switched on can be reset.

In this example, the peripheral circuits of the imaging device 101include the plurality of inverting amplifiers 22 provided for therespective columns of unit pixel cells 14. As schematically shown inFIG. 1, negative input terminals of the inverting amplifiers 22 areconnected to the respective vertical signal lines 17. Output terminalsof the inverting amplifiers 22 are connected to respective feedbacklines 23 provided for their respective columns of unit pixel cells 14.Each feedback line 23 is connected to unit pixel cells 14 that areconnected to the negative input terminal of a corresponding invertingamplifier 22.

The output terminals of the inverting amplifiers 22 are connectedthrough the feedback lines 23 to the drain or source electrodes of therespective reset transistors 12, which electrodes are not connected toany pixel electrodes 50. In each unit pixel cell 14, when the addresstransistor 13 is electrically continuous with the reset transistor 12, afeedback path is formed between the unit pixel cell 14 with the addresstransistor 13 and the reset transistor 12 switched on and acorresponding inverting amplifier 22. In this case, the potential of apositive input terminal of the inverting amplifier 22 is fixed to aprescribed potential. When the feedback path is formed, the voltage of acorresponding vertical signal line 17 converges to an input voltage Vrefto the negative input terminal of the inverting amplifier 22. In otherwords, when the feedback path is formed, the voltage of the chargestorage node 24 is reset such that the voltage of the vertical signalline 17 is equal to Vref. The voltage Vref used may be any voltagewithin the range of from the power source voltage to the ground voltage(0 V). The inverting amplifiers 22 may be referred to also as feedbackamplifiers.

(Device Structure of Unit Pixel Cell)

FIG. 2 shows an exemplary device structure of a unit pixel cell 14 inthe imaging device 101 according to the embodiment of the presentdisclosure.

As described above, the unit pixel cell 14 includes the charge detectioncircuit 25 and the photoelectric converter 10. The amplificationtransistor 11, the reset transistor 12, and the address transistor 13 inthe charge detection circuit 25 are formed on a semiconductor substrate31. The semiconductor substrate 31 includes, for example, n-typeimpurity regions 41A, 41B, 41C, 41D, and 41E. In this example,interlayer insulating layers 43A, 43B, and 43C are stacked on thesurface of the semiconductor substrate 31, and the photoelectricconverter 10 is disposed on the interlayer insulating layer 43C.Although not shown in FIG. 2, the vertical scanning circuit 15, thehorizontal signal reading circuit 20, the column signal processingcircuits 19, the load circuits 18, the inverting amplifiers 22, and thevoltage application circuit 60 may be formed on the semiconductorsubstrate 31. It will be appreciated that part or all of thesecomponents may be disposed on a substrate different from thesemiconductor substrate 31.

The semiconductor substrate 31 is, for example, a p-type siliconsubstrate. The “semiconductor substrate” in the present specification isnot limited to a substrate formed entirely of a semiconductor and maybe, for example, an insulating substrate having a semiconductor layer onits surface to be irradiated with light. In the following description,the amplification transistor 11, the reset transistor 12, and theaddress transistor 13 exemplified are N-channel MOSFETs.

The amplification transistor 11 includes the n-type impurity regions 41Cand 41D serving as drain and source regions, respectively, a gateinsulating layer 38B located on the semiconductor substrate 31, and agate electrode 39B located on the gate insulating layer 38B. Althoughnot illustrated in FIG. 2, one of the power source lines 21 describedabove is connected to the n-type impurity region 41C serving as thedrain region of the amplification transistor 11.

The reset transistor 12 includes the n-type impurity regions 41B and 41Aserving as drain and source regions, respectively, a gate insulatinglayer 38A located on the semiconductor substrate 31, and a gateelectrode 39A located on the gate insulating layer 38A. Although notillustrated in FIG. 2, one of the feedback lines 23 described above isconnected to the n-type impurity region 41A serving as the source regionof the reset transistor 12.

The address transistor 13 includes the n-type impurity regions 41D and41E serving as drain and source regions, respectively, a gate insulatinglayer 38C located on the semiconductor substrate 31, and a gateelectrode 39C located on the gate insulating layer 38C. In this example,the amplification transistor 11 and the address transistor 13 share then-type impurity region 41D. Since the n-type impurity region 41D isshared, the amplification transistor 11 and the address transistor 13are connected in series. Although not illustrated in FIG. 2, one of thevertical signal lines 17 described above is connected to the n-typeimpurity region 41E serving as the source region of the addresstransistor 13.

The gate insulating layer 38B of the amplification transistor 11, thegate insulating layer 38A of the reset transistor 12, and the gateinsulating layer 38C of the address transistor 13 are typically disposedin the same layer. Similarly, the gate electrode 39B of theamplification transistor 11, the gate electrode 39A of the resettransistor 12, and the gate electrode 39C of the address transistor 13are typically disposed in the same layer.

In the semiconductor substrate 31, element isolation regions 42 areprovided between the unit pixel cell 14 and its adjacent unit pixelcells 14 and between the amplification transistor 11 and the resettransistor 12. The element isolation regions 42 electrically isolate theunit pixel cell 14 from its adjacent unit pixel cells 14. Moreover,leakage of signal charges stored in the charge storage node 24 isprevented.

In the structure exemplified in FIG. 2, a connection member 48 thatelectrically connects the pixel electrode 50 of the photoelectricconverter 10 to the gate electrode 39B of the amplification transistor11 is disposed in the layered structure including the interlayerinsulating layers 43A, 43B, and 43C. In this case, the connection member48 includes wiring lines 46A, 46B, and 46C, plugs 47A, 47B, and 47C, andcontact plugs 45A and 45B. As illustrated, the plug 47C connects thepixel electrode 50 to the wiring line 46C, and the contact plug 45Bconnects the gate electrode 39B to the wiring line 46A.

The contact plug 45A that connects the n-type impurity region 41B of thereset transistor 12 to the wiring line 46A is disposed in the interlayerinsulating layer 43A. The contact plug 45A and the contact plug 45B areconnected to each other through the wiring line 46A. Specifically, inthis case, the pixel electrode 50 is electrically connected also to then-type impurity region 41B in the semiconductor substrate 31.

As described above, the signal charges collected by the pixel electrode50 are stored in the charge storage node 24. The charge storage node 24includes, as a part thereof, the connection member 48 that electricallyconnects the pixel electrode 50 to the gate electrode 39B of theamplification transistor 11. In this case, since the pixel electrode 50is electrically connected also to the n-type impurity region 41B in thesemiconductor substrate 31, the n-type impurity region 41B functions asa charge storage region that stores the signal charges. Specifically,the charge storage node 24 includes the pixel electrode 50, the gateelectrode 39B, and the n-type impurity region 41B and further includesthe plugs 47A, 47B, and 47C, the contact plugs 45A and 45B, and thewiring lines 46A, 46B, and 46C that electrically connect the pixelelectrode 50, the gate electrode 39B, and the n-type impurity region41B. The charge detection circuit 25 including, as a part thereof, theamplification transistor 11 having the gate electrode 39B connected tothe connection member 48 detects the signal charges collected by thepixel electrode 50 and stored in the charge storage node 24 and outputsa signal voltage.

The photoelectric converter 10 on the interlayer insulating layer 43Cincludes the light-transmitting counter electrode 52, the photoelectricconversion structure 51, and the pixel electrode 50 located closer tothe semiconductor substrate 31 than the counter electrode 52. Asschematically shown in FIG. 2, the photoelectric conversion structure 51is sandwiched between the counter electrode 52 and the pixel electrode50. Light having passed through the counter electrode 52 is incident onthe photoelectric conversion structure 51. The details of the structureof the photoelectric conversion structure 51 will be described later.

A transparent conducting oxide (TCO) with high near-infrared and visiblelight transmittance and small resistance may be used as the material ofthe counter electrode 52. Examples of the TCO used include indium tinoxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO),fluorine-doped tin oxide (FTO), SnO₂, TiO₂, and ZnO₂. A metal thin filmsuch as an Au thin film may be used as the counter electrode 52. Theterms “light-transmitting” and “transparent as used herein mean that atleast part of light in the detection wavelength range is allowed to passthrough. Although not shown in FIG. 2, one of the bias voltage lines 16described above is connected to the counter electrode 52. Duringoperation of the imaging device 101, a prescribed bias voltage isapplied to the counter electrode 52 through the bias voltage line 16.

The pixel electrode 50 disposed on the interlayer insulating layer 43Cis formed from a metal such as aluminum or copper or polysilicon dopedwith impurities to impart electric conductivity. The pixel electrodes 50in the plurality of unit pixel cells 14 are spaced apart from eachother, and therefore the pixel electrode 50 of each unit pixel cell 14is electrically isolated from pixel electrodes 50 of its adjacent unitpixel cells 14.

As shown in FIG. 2, the unit pixel cell 14 may have an optical filter 53that faces the counter electrode 52 of the photoelectric converter 10.The optical filter 53 selectively passes through or blocks light in aspecific wavelength range that is contained in the light incident on theunit pixel cell 14. A protective layer may be disposed between thecounter electrode 52 and the optical filter 53. A microlens 54 may bedisposed on the optical filter 53 or the protective layer.

The imaging device 101 can be manufactured using a general semiconductormanufacturing process. In particular, when a silicon substrate is usedas the semiconductor substrate 31, the imaging device 101 can bemanufactured using various silicon semiconductor processes.

(Photoelectric Conversion Structure)

FIG. 3 shows an example of a cross-sectional structure of thephotoelectric converter 10. As described above, the photoelectricconverter 10 includes the pixel electrode 50, the counter electrode 52,and the photoelectric conversion structure 51 sandwiched therebetween.Typically, the photoelectric conversion structure 51 includes aplurality of organic material-containing layers. In the structureexemplified in FIG. 3, the photoelectric conversion structure 51includes a layered structure including a first photoelectric conversionlayer 511 and a second photoelectric conversion layer 512. Asillustrated, in this example, the second photoelectric conversion layer512 is located between the first photoelectric conversion layer 511 andthe counter electrode 52.

In the structure exemplified in FIG. 3, the photoelectric conversionstructure 51 further includes an electron blocking layer 515 and a holetransport layer 513 that are disposed between the first photoelectricconversion layer 511 and the pixel electrode 50. The electron blockinglayer 515 is adjacent to the pixel electrode 50, and the hole transportlayer 513 is adjacent to the first photoelectric conversion layer 511.The photoelectric conversion structure 51 further includes an electrontransport layer 514 and a hole blocking layer 516 that are disposedbetween the second photoelectric conversion layer 512 and the counterelectrode 52. The hole blocking layer 516 is adjacent to the counterelectrode 52, and the electron transport layer 514 is adjacent to thesecond photoelectric conversion layer 512.

In this structure, positive and negative charges are generated byphotoelectric conversion, and the positive charges (e.g., holes) aredetected as signal charges. During operation of the imaging device 101,a switching voltage is supplied from the voltage application circuit 60to the counter electrode 52 such that the potential of the counterelectrode 52 is higher than the potential of the pixel electrode 50. Forexample, the voltage application circuit 60 supplies, to the counterelectrode 52, one of the first voltage VA and the second voltage VB withdifferent absolute values (|VB|>|VA|) as the switching voltage. Which ofthe first voltage VA and the second voltage VB is supplied to thecounter electrode 52 is determined, for example, by an instruction fromthe operator of the imaging device 101 or an instruction from anothercontroller included in the imaging device 101. Specific examples of theoperation of the imaging device 101 will be described later.

The electron blocking layer 515 shown in FIG. 3 is provided for thepurpose of reducing a dark current caused by injection of electrons fromthe pixel electrode 50, and the hole blocking layer 516 is provided forthe purpose of reducing a dark current caused by injection of holes fromthe counter electrode 52. The hole transport layer 513 is provided forthe purpose of efficiently transporting positive charges generated inthe first photoelectric conversion layer 511 and/or the secondphotoelectric conversion layer 512 to the pixel electrode 50. Theelectron transport layer 514 is provided for the purpose of efficientlytransporting electrons generated in the first photoelectric conversionlayer 511 and/or the second photoelectric conversion layer 512 to thecounter electrode 52. The materials forming the electron blocking layer515, the hole blocking layer 516, the hole transport layer 513, and theelectron transport layer 514 may be selected from known materials inconsideration of bonding strength with adjacent layers, stability, thedifference in ionization potential, the difference in electron affinity,etc. At least one of the materials forming the electron blocking layer515, the hole blocking layer 516, the hole transport layer 513, and theelectron transport layer 514 may be the material for forming the firstphotoelectric conversion layer 511 or the material for forming thesecond photoelectric conversion layer 512.

The first photoelectric conversion layer 511 contains a first material(typically a semiconductor material). The second photoelectricconversion layer 512 contains a second material (typically asemiconductor material). In some aspects, the impedance of the firstphotoelectric conversion layer 511 per unit thickness differs from theimpedance of the second photoelectric conversion layer 512 per unitthickness. In some aspects of the present disclosure, the impedance ofthe first photoelectric conversion layer 511 per unit thickness islarger than the impedance of the second photoelectric conversion layer512 per unit thickness. The impedance of each photoelectric conversionlayer depends on its thickness. When a photoelectric conversion layercontains a plurality of materials, its impedance depends also on thevolume ratio of these materials in the photoelectric conversion layer.In the embodiment of the present disclosure, among the plurality ofphotoelectric conversion layers included in the layered structure, alayer having a lager impedance can be used as the first photoelectricconversion layer 511. The impedance of the first photoelectricconversion layer and the impedance of the second photoelectricconversion layer may be impedances at a frequency of 1 Hz with the firstand second photoelectric conversion layers not irradiated with light.

(Switching of Spectral Sensitivity Characteristics by Changing BiasVoltage Using Difference in Impedance)

Suppose that the photoelectric conversion structure 51 includes thefirst photoelectric conversion layer 511 and the second photoelectricconversion layer 512 that differ in impedance. In this case, when a biasvoltage is applied between the pixel electrode 50 and the counterelectrode 52, voltages (or electric fields) proportional to theimpedances of the first and second photoelectric conversion layers 511and 512 are applied to the first and second photoelectric conversionlayers 511 and 512. The inventors have found that, by changing the biasvoltage applied to a photoelectric conversion structure includingphotoelectric conversion layers with different impedances, externalquantum efficiency (E. Q. E.) in a certain wavelength range can bechanged. In other words, the inventors have found that the spectralsensitivity characteristics of a unit pixel cell having the abovephotoelectric conversion structure in the photoelectric converter can beelectrically changed. For example, an increase in the external quantumefficiency of the photoelectric conversion structure 51 at a wavelengthcorresponding to the absorption peak of the second material when thesecond voltage VB is applied with respect to the external quantumefficiency when the first voltage VA is applied can be larger than anincrease in the external quantum efficiency of the photoelectricconversion structure 51 at a wavelength corresponding to the absorptionpeak of the first material when the second voltage VB is applied withrespect to the external quantum efficiency when the first voltage VA isapplied.

Suppose, for example, that Z1>Z2, where Z1 is the impedance of the firstphotoelectric conversion layer 511, and Z2 is the impedance of thesecond photoelectric conversion layer 512. In this case, the voltageapplied to the first photoelectric conversion layer 511 is larger thanthe voltage applied to the second photoelectric conversion layer 512.Therefore, even when the bias between the pixel electrode 50 and thecounter electrode 52 is small, i.e., even when the first voltage VA issupplied to the counter electrode 52, an electric field sufficient toallow the charges generated by photoelectric conversion to move to theelectrodes can be applied to the first photoelectric conversion layer511. Specifically, the positive and negative charges generated byphotoelectric conversion can reach the pixel electrode 50 and thecounter electrode 52, respectively. The signal charges generated byirradiation of the first photoelectric conversion layer 511 with lightare collected by the pixel electrode 50 and stored in the charge storageregion.

The electric field applied to the second photoelectric conversion layer512 is lower than the electric field applied to the first photoelectricconversion layer 511. Therefore, when the first voltage VA with arelatively small absolute value is supplied to the counter electrode 52,the electric field applied to the second photoelectric conversion layer512 may be lower than the electric field necessary to allow the signalcharges generated by irradiation of the second photoelectric conversionlayer 512 with light to reach the pixel electrode 50. If the signalcharges do not reach the pixel electrode 50, the signal chargesgenerated in the second photoelectric conversion layer 512 are notstored in the charge storage region. Therefore, the unit pixel cell 14does not exhibit sufficient sensitivity to light in a wavelength rangecorresponding to the absorption spectrum of the materials (particularlythe second material) forming the second photoelectric conversion layer512.

When the voltage applied between the counter electrode 52 and the pixelelectrode 50 is increased, the voltage applied to the secondphotoelectric conversion layer 512 increases. Specifically, by supplyingthe second voltage VB with a larger absolute value to the counterelectrode 52, the electric field applied to the second photoelectricconversion layer 512 increases, and the signal charges are allowed toreach the pixel electrode 50. Therefore, the unit pixel cell 14 exhibitssensitivity not only to light in a wavelength range corresponding to theabsorption spectrum of the materials (particularly the first material)forming the first photoelectric conversion layer 511 but also to lightin a wavelength range corresponding to the absorption spectrum of thematerials (particularly the second material) forming the secondphotoelectric conversion layer 512.

As described above, when the layered structure including the firstphotoelectric conversion layer 511 and the second photoelectricconversion layer 512 having a lower impedance than the firstphotoelectric conversion layer 511 is used, the spectral sensitivitycharacteristics can be changed by changing the bias voltage supplied tothe counter electrode 52. The ratio of the impedance of the firstphotoelectric conversion layer 511 to the impedance of the secondphotoelectric conversion layer 512 is typically within the range of from100 to 10¹⁰. According to studies by the inventors, when the ratio ofthe impedance of the first photoelectric conversion layer 511 to theimpedance of the second photoelectric conversion layer 512 is, forexample, 44 or more, the spectral sensitivity characteristics can bechanged by changing the bias voltage. The ratio of the impedance of thefirst photoelectric conversion layer 511 to the impedance of the secondphotoelectric conversion layer 512 may be 190 or more. The impedance ofthe first photoelectric conversion layer and the impedance of the secondphotoelectric conversion layer may be impedances at a frequency of 1 Hzwith the first and second photoelectric conversion layers not irradiatedwith light.

As will be described below, a combination of the first and secondmaterials may be, for example, a combination of a material having a highabsorption coefficient in the visible range and a material having a highabsorption coefficient in the infrared range. With such a combination ofmaterials, an imaging device that can acquire one or both of informationabout the illuminance of visible light and information about theilluminance of infrared light can be provided.

Typically, each of the first photoelectric conversion layer 511 and thesecond photoelectric conversion layer 512 contains electron-donatingp-type molecules and electron-accepting n-type molecules.

The first and second materials used may be, for example,electron-donating molecules. Typical examples of the electron-donatingmolecules include organic p-type semiconductors, and representativeexamples of such organic p-type semiconductors include hole transportingorganic materials having electron-donating properties. Examples of theorganic p-type semiconductor include: triarylamine compounds such asDTDCTB(2-{[7-(5-N,N-ditolylaminothiophen-2-yl)-2,1,3-benzothiadiazol-4-yl]methylene}malononitrile);benzidine compounds; pyrazoline compounds; styrylamine compounds;hydrazone compounds; triphenylmethane compounds; carbazole compounds;polysilane compounds; thiophene compounds such as α-sexithiophene(hereinafter referred to as “α-6T”) and P3HT (poly(3-hexylthiophene));phthalocyanine compounds; cyanine compounds; merocyanine compounds;oxonol compounds; polyamine compounds; indole compounds; pyrrolecompounds; pyrazole compounds; polyarylene compounds; condensed aromaticcarbocyclic compounds (naphthalene derivatives, anthracene derivatives,phenanthrene derivatives, tetracene derivatives such as rubrene, pyrenederivatives, perylene derivatives, and fluoranthene derivatives); andmetal complexes having as ligands nitrogen-containing heterocyclecompounds. Examples of the phthalocyanine compound include copperphthalocyanine (CuPc), subphthalocyanine (SubPc), aluminum chloridephthalocyanine (ClAlPc), Si(OSiR₃)₂Nc (where R represents an alkyl grouphaving 1 to 18 carbon atoms, and Nc represents naphthalocyanine), tinnaphthalocyanine (SnNc), and lead phthalocyanine (PbPc). Theelectron-donating organic semiconductor is not limited to the abovecompounds, and any organic compound having a lower ionization potentialthan an organic compound used as the n-type (electron-accepting)compound may be used as the electron-donating organic semiconductor. Theionization potential is the difference between the vacuum level and theenergy level of the highest occupied molecular orbital (HOMO).

Typical examples of the electron-accepting molecules include organicn-type semiconductors, and representative examples of such organicp-type semiconductors include electron transporting organic compoundshaving electron-accepting properties. Examples of the organic n-typesemiconductor include: fullerenes such as C₆₀ and C₇₀; fullerenederivatives such as phenyl-C₆₁-butyric acid methyl ester (PCBM);condensed aromatic carbocyclic compounds (naphthalene derivatives,anthracene derivatives, phenanthrene derivatives, tetracene derivatives,pyrene derivatives, perylene derivatives, and fluoranthene derivatives);5 to 7-membered heterocyclic compounds containing a nitrogen atom, anoxygen atom, or a sulfur atom (such as pyridine, pyrazine, pyrimidine,pyridazine, triazine, quinoline, quinoxaline, quinazoline, phthalazine,cinnoline, isoquinoline, pteridine, acridine, phenazine, phenanthroline,tetrazole, pyrazole, imidazole, thiazole, oxazole, indazole,benzimidazole, benzotriazole, benzoxazole, benzothiazole, carbazole,purine, triazolopyridazine, triazolopyrimidine, tetrazaindene,oxadiazole, imidazopyridine, pyrrolidine, pyrrolopyridine,thiadiazolopyridine, dibenzazepine, and tribenzazepine);subphthalocyanine (SubPc); polyarylene compounds; fluorene compounds;cyclopentadiene compounds; silyl compounds; perylenetetracarboxylicdiimide compounds (PTCDI), and metal complexes having as ligandsnitrogen-containing heterocycle compounds. The electron-acceptingorganic semiconductor is not limited to the above compounds, and anyorganic compound having a higher electron affinity than an organiccompound used as the p-type (electron-donating) compound may be used asthe electron-accepting organic semiconductor. The electron affinity isthe difference between the vacuum level and the energy level of thelowest unoccupied molecular orbital (LUMO).

FIG. 4A shows the structural formula of SnNc. FIG. 4B shows thestructural formula of DTDCTB, and FIG. 4C shows the structural formulaof C₇₀. The above examples are not limitations, and any organic compoundor organic molecules that can be formed into a film by a wet or drymethod can be used as the material forming the first photoelectricconversion layer 511 or the material forming the second photoelectricconversion layer 512, irrespective of whether they are low-molecularweight molecules or high-molecular weight molecules.

By using materials suitable for a detection wavelength range as thefirst and second materials, the photoelectric conversion structure 51obtained can have sensitivity in the desired wavelength range. Forexample, a material having an absorption peak in the visible range maybe used as the first material, and a material having an absorption peakin the infrared range may be used as the second material. DTDCTBdescribed above has an absorption peak at a wavelength of about 700 nm,and CuPc and SubPc have absorption peaks at wavelengths of about 620 nmand about 580 nm, respectively. Si(OSiR₃)₂Nc has an absorption peak at awavelength of about 790 nm. Rubrene has an absorption peak at awavelength of about 530 nm, and α-6T has an absorption peak at awavelength of about 440 nm. The absorption peaks of these materials fallwithin the visible wavelength range, and these materials can be used as,for example, the first material. SnNc has an absorption peak at awavelength of about 870 nm, and ClAlPc has an absorption peak at awavelength of about 750 nm. The absorption peaks of these materials fallwithin the infrared wavelength range, and these materials can be usedas, for example, the second material. Of course, a material having anabsorption peak in the infrared range may be used as the first material,and a material having an absorption peak in the visible range may beused as the second material.

In the present specification, the visible wavelength range is awavelength range of, for example, 380 nm or more and less than 750 nm,and the infrared wavelength range is a wavelength range of, for example,750 nm or more. The near-infrared wavelength range is a wavelength rangeof, for example, 750 nm or more and less than 1,400 nm. In the presentspecification, all electromagnetic waves including infrared rays andultraviolet rays are referred to as “light” for the sake of convenience.For example, when a material having an absorption peak at a firstwavelength in the visible range is used as the first material and amaterial having an absorption peak at a second wavelength in theinfrared range is used as the second material, the sensitivity in theinfrared range can be electrically changed.

Suppose, for example, that the impedance Z1 of the first photoelectricconversion layer 511 using, as the first material, a material having ahigh absorption coefficient for visible light is higher than theimpedance Z2 of the second photoelectric conversion layer 512 using, asthe second material, a material having a high absorption coefficient forinfrared light (Z1>Z2). The impedance of the first photoelectricconversion layer and the impedance of the second photoelectricconversion layer may be impedances at a frequency of 1 Hz with the firstand second photoelectric conversion layers not irradiated with light. Inthis case, when the voltage applied between the counter electrode 52 andthe pixel electrode 50 is equal to or lower than a threshold value, thephotoelectric converter 10 exhibits higher sensitivity in the visiblerange. Therefore, image signals using visible light can be obtained.When the voltage applied between the counter electrode 52 and the pixelelectrode 50 is higher than the threshold value, the photoelectricconverter 10 exhibits sensitivity in the visible range and the infraredrange. This allows image signals using visible light and infrared lightto be acquired. In other words, as for the voltage applied between thecounter electrode 52 and the pixel electrode 50, the relation |V1|<|V2|holds, where V1 is a voltage at which an image can be acquired usingvisible light, and V2 is a voltage at which an image can be acquiredusing visible light and infrared light.

Suppose, in contrast, that the impedance Z1 of the first photoelectricconversion layer 511 is lower than the impedance Z2 of the secondphotoelectric conversion layer 512 (Z1<Z2). In this case, when thevoltage applied between the counter electrode 52 and the pixel electrode50 is equal to or lower than a threshold value, the photoelectricconverter 10 can have higher sensitivity in the infrared range.Therefore, the imaging device 101 can acquire image signals usinginfrared light. When the voltage applied between the counter electrode52 and the pixel electrode 50 exceeds the threshold value, thephotoelectric converter 10 has sensitivity in the visible range and theinfrared light. This allows image signals using visible light andinfrared light to be acquired. In this case, as for the voltage appliedbetween the counter electrode 52 and the pixel electrode 50, therelation |V3|<|V4| holds, where V3 is a voltage at which an image can beacquired using visible light, and V4 is a voltage at which an image canbe acquired using visible light and infrared light. It should be notedthat the image acquirable wavelength band can be changed by changing thevoltage applied between the counter electrode 52 and the pixel electrode50.

When the first photoelectric conversion layer 511 and the secondphotoelectric conversion layer 512 each contain only one type of organicmaterial, they may not have the desired sensitivity characteristics. Inthis case, one or both of the first photoelectric conversion layer 511and the second photoelectric conversion layer 512 may be formed from amixture of two or more organic materials. Alternatively, one or both ofthe first photoelectric conversion layer 511 and the secondphotoelectric conversion layer 512 may be formed by stacking two or morelayers containing different organic materials. The first photoelectricconversion layer 511 and/or the second photoelectric conversion layer512 may be, for example, a bulk heterojunction structure layer includinga p-type semiconductor and an n-type semiconductor. The bulkheterojunction structure is described in detail in Japanese Patent No.5553727. For reference purposes, the entire contents of Japanese PatentNo. 5553727 are incorporated herein by reference. The firstphotoelectric conversion layer 511 and the second photoelectricconversion layer 512 may contain an inorganic semiconductor materialsuch as amorphous silicon. The first photoelectric conversion layer 511and/or the second photoelectric conversion layer 512 may include a layerformed from an organic material and a layer formed from an inorganicmaterial.

FIG. 5 shows another example of the cross-sectional structure of thephotoelectric converter 10. The photoelectric conversion structure 51Ashown in FIG. 5 includes the first photoelectric conversion layer 511, amixture layer 510, and the second photoelectric conversion layer 512.The mixture layer 510 contains at least the first and second materialsand is located between the first photoelectric conversion layer 511 andthe second photoelectric conversion layer 512. FIG. 5 and also FIG. 3are merely schematic diagrams, and the boundaries between the layersincluded in the photoelectric conversion structure may not be strictlydefined. This is also the case for other cross-sectional views in thepresent disclosure. According to studies by the inventors, when thelayer structure including the first photoelectric conversion layer 511and the second photoelectric conversion layer 512 is not formed and onlya layer containing a mixture of the first and second materials used aselectron-accepting organic semiconductors and an electron-donatingorganic semiconductor is used as the photoelectric conversion structure,the external quantum efficiency in both the wavelength rangecorresponding to the absorption spectrum of the first material and thewavelength range corresponding to the absorption spectrum of the secondmaterial can increase as the bias is increased. Therefore, the effect ofchanging the spectral sensitivity by changing the bias voltage may notbe obtained.

As described above, the structure of the photoelectric converter 10 isnot limited to the structure shown in FIG. 3. For example, thearrangement of the first photoelectric conversion layer 511 and thesecond photoelectric conversion layer 512 may be reversed from thearrangement shown in FIGS. 3 and 5. Positive and negative charges aregenerated in the photoelectric conversion structure 51. When thenegative charges (typically electrons) are used as the signal charges, ahole blocking layer and an electron transport layer may be used insteadof the electron blocking layer 515 and the hole transport layer 513, anda hole transport layer and an electron blocking layer may be usedinstead of the electron transport layer 514 and the hole blocking layer516.

(Switching of Spectral Sensitivity Characteristics by Changing BiasVoltage Using Difference in Ionization Potential)

The inventors have also found that, even when the impedance of the firstphotoelectric conversion layer 511 is equal to or lower than theimpedance of the second photoelectric conversion layer 512, the spectralsensitivity characteristics can be changed by changing the applied biasvoltage when the difference in ionization potential between the firstand second materials is somewhat large.

FIG. 6 is an energy diagram in a still another structural example of thephotoelectric converter. Rectangles in FIG. 6 each schematically showthe LUMO and HOMO of a material. Numerical values near the upper andlower sides of each rectangle represent the electron affinity andionization potential of a corresponding material. Thick horizontal linesin FIG. 6 schematically represent exemplary Fermi levels of the counterelectrode 52 and the pixel electrode 50.

In the structure shown in FIG. 6, the photoelectric conversion structure51B has a layered structure in which the electron blocking layer 515,the first photoelectric conversion layer 511, and the secondphotoelectric conversion layer 512 are stacked from the pixel electrode50 toward the counter electrode 52. In this example, rubrene, SnNc, andbis(carbazolyl)benzodifuran (CZBDF), which is an am bipolar organicsemiconductor, are used as the first material, the second material, andthe material of the electron blocking layer 515, respectively. FIG. 7shows the structural formula of CZBDF. As schematically shown in FIG. 6,the first photoelectric conversion layer 511 and the secondphotoelectric conversion layer 512 each contain C₇₀ serving as anelectron-accepting organic semiconductor. The first photoelectricconversion layer 511 in this example generates charge pairs throughphotoelectric conversion when irradiated with visible light, and thesecond photoelectric conversion layer 512 generates charge pairs throughphotoelectric conversion when irradiated with infrared light. In FIG. 6,open circles “O” and filled circles “●” represent positive and negativecharges, respectively, generated by photoelectric conversion.

As has been described above, when positive charges are collected by thepixel electrode 50, the first voltage VA is supplied to, for example,the counter electrode 52, so that the potential of the counter electrode52 is higher than the potential of the counter electrode 52. In thisstate, when visible light is incident on the first photoelectricconversion layer 511 and positive and negative charges are generated inthe first photoelectric conversion layer 511, the positive charges arecollected by the pixel electrode 50. Specifically, in this state, thesignal charges generated by the irradiation with visible light arestored in the charge storage region, and the unit pixel cell 14 hassensitivity in the visible wavelength range. The negative chargestransfer from the LUMO level of rubrene to the LUMO level of C₇₀ andmove toward the counter electrode 52 along the electric field betweenthe pixel electrode 50 and the counter electrode 52. Since C₇₀ is usedas the electron-accepting organic semiconductor for the firstphotoelectric conversion layer 511 and also for the second photoelectricconversion layer 512, the negative charges transferred to the LUMO levelof C₇₀ can move to the counter electrode 52 and be collected by thecounter electrode 52.

Suppose that infrared light is incident on the second photoelectricconversion layer 512 and positive and negative charges are generated inthe second photoelectric conversion layer 512. The positive charges movetoward the pixel electrode 50 along the electric field between the pixelelectrode 50 and the counter electrode 52. However, as shown in FIG. 6,the ionization potential of rubrene is higher than the ionizationpotential of SnNc, and a potential barrier for the positive charges isformed between the HOMO level of SnNc and the HOMO level of rubrene.Therefore, when the bias between the pixel electrode 50 and the counterelectrode 52 is small, the positive charges cannot overcome thepotential barrier and do not reach the pixel electrode 50. This meansthat the unit pixel cell 14 does not have sensitivity in the infraredwavelength range.

When the bias between the pixel electrode 50 and the counter electrode52 is increased to impart, to the positive charges, energy sufficient toovercome the potential barrier, the positive charges can overcome thepotential barrier and reach the pixel electrode 50. Specifically, forexample, by supplying the second voltage VB having a larger absolutevalue than the first voltage VA to the counter electrode 52 from thevoltage application circuit 60, the positive charges generated in thesecond photoelectric conversion layer 512 can be collected by the pixelelectrode 50. In other words, by changing the bias voltage supplied tothe unit pixel cell 14, the unit pixel cell 14 can have sensitivity inthe infrared wavelength range. In this state, the unit pixel cell 14 hassensitivity in the visible wavelength range and also in the infraredwavelength range.

According to studies by the inventors, when the absolute value of theionization potential of the first material is larger by at least 0.2 eVthan the absolute value of the ionization potential of the secondmaterial, the effect of changing the spectral sensitivitycharacteristics by changing the bias voltage can be obtained. In thestructure in which the second photoelectric conversion layer 512 islocated between the first photoelectric conversion layer 511 and thecounter electrode 52 as exemplified in FIG. 6, a switching voltage thatcauses the potential of the counter electrode 52 to be higher than thepotential of the pixel electrode 50 is used.

As described above, when the absolute value of the ionization potentialof the first material is larger by at least 0.2 eV than the absolutevalue of the ionization potential of the second material, the spectralsensitivity characteristics of the unit pixel cell 14 can beelectrically changed even when the impedance of the first photoelectricconversion layer 511 is equal to or lower than the impedance of thesecond photoelectric conversion layer 512. In this case, the impedanceof the first photoelectric conversion layer 511 may be larger than theimpedance of the second photoelectric conversion layer 512.

FIG. 8 is an energy diagram of a photoelectric conversion structure 51Cin which the positions of the first photoelectric conversion layer 511and the second photoelectric conversion layer 512 in the photoelectricconversion structure 51B shown in FIG. 6 are exchanged with each other.

In the structure shown in FIG. 8, no potential barrier for positivecharges moving toward the pixel electrode 50 is present between thefirst photoelectric conversion layer 511 and the second photoelectricconversion layer 512. Therefore, by using a bias voltage that causes thepotential of the counter electrode 52 to be higher than the potential ofthe pixel electrode 50, positive charges generated in the firstphotoelectric conversion layer 511 and also positive charges generatedin the second photoelectric conversion layer 512 can reach the pixelelectrode 50, as schematically shown by open circles and arrows in FIG.8. Therefore, the effect of changing the spectral sensitivitycharacteristics by using the difference in ionization potential betweenthe first photoelectric conversion layer 511 and the secondphotoelectric conversion layer 512 is not expected. However, when theimpedance of the first photoelectric conversion layer 511 is larger thanthe impedance of the second photoelectric conversion layer 512, thefunction of changing the spectral sensitivity characteristics bychanging the bias voltage can be obtained even for the above stackingorder. The impedance of the first photoelectric conversion layer and theimpedance of the second photoelectric conversion layer may be impedancesat a frequency of 1 Hz with the first and second photoelectricconversion layers not irradiated with light.

According to at least one of the embodiments of the present disclosure,by using at least two bias voltages with different absolute values, thespectral sensitivity characteristics of the unit pixel cell 14 can beelectrically changed as described above. Positive and negative chargesare generated by photoelectric conversion. When the negative charges areused as signal charges, a hole blocking layer is used instead of theelectron blocking layer 515, and a bias voltage that causes thepotential of the counter electrode 52 to be lower than the potential ofthe pixel electrode 50 is supplied. In both the examples in FIGS. 6 and8, no potential barrier for electrons is formed between the firstphotoelectric conversion layer 511 and the second photoelectricconversion layer 512. When the impedance of the first photoelectricconversion layer 511 is larger than the impedance of the secondphotoelectric conversion layer 512, the spectral sensitivitycharacteristics can be changed by changing the bias voltage. Theimpedance of the first photoelectric conversion layer and the impedanceof the second photoelectric conversion layer may be impedances at afrequency of 1 Hz with the first and second photoelectric conversionlayers not irradiated with light.

(Operation Examples of Imaging Device)

Next, operation examples of the imaging device 101 will be described.

FIG. 9 is a schematic cross-sectional view of two adjacent unit pixelcells in the photosensitive region. As described above with reference toFIG. 2, each of the unit pixel cells 14 in the photosensitive region mayhave an optical filter 53 facing the counter electrode 52. FIG. 9schematically shows a cross-section of a unit pixel cell 14 x having anoptical filter 330 and a cross-section of a unit pixel cell 14 y havingan optical filter 531. The optical filters 530 and 531 in the structureexemplified above are color filters that can selectively transmit lightin the visible and infrared wavelength ranges, and their wavelengthranges in which light in the visible range is selectively absorbeddiffer from each other. The optical filter 530 may be one of an R filterhaving high transmittance of red light, a G filter having hightransmittance of green light, and a B filter having high transmittanceof blue light. The optical filter 531 may be another one of the R, G,and B filters.

In the exemplified structure, a material having a high absorptioncoefficient in the visible range (e.g., DTDCTB) and a material having ahigh absorption coefficient in the infrared range (e.g., SnNc) are usedas the first material and the second material, respectively, containedin a photoelectric conversion structure 51 x of the unit pixel cell 14 xand a photoelectric conversion structure 51 y of the unit pixel cell 14y. Each of the photoelectric conversion structures 51 x and 51 yincludes a layered structure including a first photoelectric conversionlayer 511 and a second photoelectric conversion layer 512, although thelayered structure is omitted in FIG. 9 to avoid excessive complexity ofthe illustration. In the example described herein, the impedance of thefirst photoelectric conversion layer 511 is larger than the impedance ofthe second photoelectric conversion layer 512. The impedance of thefirst photoelectric conversion layer and the impedance of the secondphotoelectric conversion layer may be impedances at a frequency of 1 Hzwith the first and second photoelectric conversion layers not irradiatedwith light. Alternatively, the absolute value of the ionizationpotential of the first material is larger by at least 0.2 eV than theabsolute value of the ionization potential of the second material.

In the structure exemplified in FIG. 9, the photoelectric conversionstructure 51 x of the unit pixel cell 14 x and the photoelectricconversion structure 51 y of the unit pixel cell 14 y are formed as asingle continuous structure. When a plurality of unit pixel cells 14share a common photoelectric conversion structure as in this example,the photoelectric conversion structures 51 of the plurality of unitpixel cells 14 can be formed at once, so an increase in complexity ofthe manufacturing process can be avoided.

In this example, a counter electrode 52 x of the unit pixel cell 14 xand a counter electrode 52 y of the unit pixel cell 14 y are each formedas a part of a single continuous electrode. In this structure, thecounter electrodes 52 of the plurality of unit pixel cells 14 can beformed at once, so an increase in complexity of the manufacturingprocess can be avoided. When the single continuous electrode is used toform the counter electrodes 52 of the plurality of unit pixel cells 14,the same switching voltage can be applied to the counter electrodes 52of the plurality of unit pixel cells 14 while an increase in complexityof wiring lines is avoided. The counter electrode 52 of the unit pixelcells 14 may be spaced apart and electrically isolated from each other.In this case, mutually different switching voltages may be suppliedindependently to one unit pixel cell 14 and another unit pixel cell 14.

FIG. 10 shows an example of the appearance of the photosensitive regionincluding the unit pixel cells 14 x and 14 y shown in FIG. 9, thephotosensitive region being viewed in a direction normal to thesemiconductor substrate 31. In this example, a Bayer pattern colorfilter array is used. Specifically, one of the R, G, and B filters usedas the optical filters 53 is disposed in each unit pixel cell 14 so asto face its counter electrode 52. FIG. 10 shows nine unit pixel cells14. Here, attention is given to a pixel block 14BK including four unitpixel cells 14 arranged in a 2×2 matrix. As shown in FIG. 10, in thestructure in which the color filters are disposed on the unit pixelcells 14, the following operation modes I and II, for example, areapplicable.

In the operation mode I, the voltage application circuit 60 generatesvoltages V_(L) and V_(H) satisfying the relation |V_(L)|<|V_(H)|, andthe voltage outputted is switched between the voltage V_(L) used as thefirst voltage and the voltage V_(H) used as the second voltage for everyframe. During application of the first voltage V_(L), the photoelectricconversion structure 51 of each unit pixel cell 14 has sensitivity inthe visible wavelength range. During application of the second voltageV_(H), the photoelectric conversion structure 51 of each unit pixel cell14 has sensitivity in the visible and infrared wavelength ranges. Inthis case, R, G, and B image signals (frames when the switching voltageis the first voltage V_(L)) and image signals using a combination of redlight and infrared light, a combination of green light and infraredlight, and a combination of blue light and infrared light (frames whenthe switching voltage is the second voltage V_(H)) are outputtedalternately for every frame from a unit pixel cell 14 having the Rfilter, unit pixel cells 14 each having the G filter, and a unit pixelcell 14 having the B filter.

The imaging device 101 may include a signal processing circuit connectedto the horizontal signal reading circuit 20 (see FIG. 1). For example,the signal processing circuit performs arithmetic processing on theimage signals from the unit pixel cells 14 to form an image. In thefirst operation mode I, the signal processing circuit can form an RGBcolor image using the output in a frame in which the first voltage V_(L)is used as the switching voltage. The signal processing circuitdetermines differences in pixel value between the same pixels in imagesof two successive frames. Signal components of the red light, the greenlight, and the blue light can thereby be removed, and an image using theinfrared light can be obtained. In this operation mode, a color image (astill image or a motion video) and an image using the infrared light (astill image or a motion video) can be obtained substantiallysimultaneously.

In the operation mode II, one of the first voltage V_(L) and the secondvoltage V_(H) is selectively supplied to each of the unit pixel cells 14according to, for example, the usage scene of the imaging device 101.For example, when the imaging device 101 is used as a security camera ora vehicle-mounted camera, the voltage application circuit 60 generatesthe first voltage V_(L) as the switching voltage during the daytime.Therefore, color images are acquired during the daytime. During thenighttime, the voltage application circuit 60 generates the secondvoltage V_(H) as the switching voltage to acquire images. In this case,the imaging device 101 acquires images using a combination of red lightand infrared light, a combination of green light and infrared light, anda combination of blue light and infrared light. During the nighttime,not much information about the visible range is contained in the imagesacquired. Therefore, images using substantially only infrared light canbe acquired. The use of infrared lighting is more effective in acquiringimages using infrared light. With the arrangement of the optical filters53 exemplified in FIG. 10, the image acquired can be switched, forexample, between an RGB image and an image using infrared light bychanging the bias applied between the pixel electrode 50 and the counterelectrode 52.

FIG. 11 shows another example of the appearance of the photosensitiveregion including the unit pixel cells 14 x and 14 y shown in FIG. 9, thephotosensitive region being viewed in a direction normal to thesemiconductor substrate 31. In this example, in contrast to thestructure shown in FIG. 10, one of the G filters in the pixel block 14BKis replaced with an infrared pass filter (IR filter). In this structurethe following operation mode III can be used.

In the operation mode III also, one of the first voltage V_(L) and thesecond voltage V_(H) is selectively supplied to each of the unit pixelcells 14 according to, for example, the usage scene of the imagingdevice 101. In the state in which the voltage application circuit 60supplies the first voltage V_(L) to the unit pixel cells 14, thephotoelectric conversion structure 51 of each unit pixel cell 14 hassensitivity in the visible wavelength range. Therefore, an RGB colorimage can be obtained using the output from unit pixel cells 14 with theR, G, or B filter disposed therein. A pixel value of the unit pixel cell14 with the IR filter disposed therein may be complemented, for example,using pixel values of unit pixel cells therearound. In the state inwhich the switching voltage supplied from the voltage applicationcircuit 60 to the unit pixel cells 14 is the second voltage V_(H), eachunit pixel cell 14 with the IR filter disposed therein outputs an imagesignal using infrared light. Therefore, by selectively acquiring imagesignals from the unit pixel cells 14 each having the IR filter disposedtherein, an image using infrared light can be formed. In this case, theunit pixel cells 14 each having the R filter disposed therein, the unitpixel cells 14 each having the G filter disposed therein, and the unitpixel cells 14 each having the B filter disposed therein output imagesignals for red light and infrared light, image signals for green lightand infrared light, and image signals for blue light and infrared light,respectively. The image signals from the unit pixel cells 14 having thecolor filters disposed therein may not be used and may be discarded.Specifically, in the operation mode III, when the voltage applicationcircuit 60 supplies the first voltage V_(L) to the unit pixel cells 14,a color image using visible light is acquired. When the second voltageV_(H) is supplied to the unit pixel cells 14, an image using infraredlight can be acquired. For example, when the imaging device 101 is usedas a security camera or a vehicle-mounted camera, the camera obtainedcan acquire color images during the daytime and images using infraredlight during the night time.

With the arrangement of the optical filters 53 exemplified in FIG. 11,unit pixel cells 14 that output RGB image signals and also unit pixelcells 14 that output image signals using infrared light can be presentin the photosensitive region. Therefore, as in the case of the operationmode I or II, the image obtained can be switched between an RGB imageand an image using infrared light.

In the operation modes I, II, and III described above, the switchingvoltage is switched between the first voltage V_(L) and the secondvoltage V_(H) according to the wavelength range used to acquire animage. However, while the same voltage is supplied from the voltageapplication circuit 60 to the plurality of unit pixel cells 14, the unitpixel cells 14 can have different spectral sensitivity characteristics.

FIG. 12 shows another example of the circuit structure of the imagingdevice according to the embodiment of the present disclosure. In thestructure exemplified in FIG. 12, a charge detection circuit 25 x of aunit pixel cell 14 x and a charge detection circuit 25 y of a unit pixelcell 14 y include resistors R1 and R2, respectively. In the exampleillustrated, each of the resistors R1 and R2 is connected between a gateelectrode of a corresponding amplification transistor 11 and a pixelelectrode 50 of a corresponding photoelectric converter 10. Theresistors R1 and R2 have different resistance values. For example, theresistance value of the resistor R1 is larger than the resistance valueof the resistor R2.

In the circuit structure exemplified in FIG. 12, when the second voltageV_(H) is applied to counter electrodes 52 x and 52 y, the voltage dropacross the resistor R1 differs from the voltage drop across the resistorR2. Therefore, even when the same voltage is applied from the voltageapplication circuit 60 to the unit pixel cells 14 x and 14 y, theeffective bias voltage applied between the pixel electrode 50 and thecounter electrode 52 in the unit pixel cell 14 x can differ from theeffective bias voltage applied between the pixel electrode 50 and thecounter electrode 52 in the unit pixel cell 14 y.

In this example, when the same second voltage V_(H) is supplied to thecounter electrodes 52 x and 52 y, the electric field applied to aphotoelectric conversion structure 51 y of the unit pixel cell 14 y islarger than the electric field applied to a photoelectric conversionstructure 51 x of the unit pixel cell 14 x. Therefore, an electric fieldlarge enough to impart sensitivity in the visible and infraredwavelength ranges can be applied to the unit pixel cell 14 y.Specifically, although the same voltage (the second voltage V_(H) inthis case) is applied to the counter electrodes 52 x and 52 y, an imagesignal for visible light (e.g., red light) can be acquired by the unitpixel cell 14 x, and an image signal for visible light (e.g., red light)and infrared light can be acquired by the unit pixel cell 14 y. When anIR filter is used instead of an optical filter 531 in the unit pixelcell 14 y, the unit pixel cell 14 y can acquire an image signal forinfrared light.

The resistors R1 and R2 are not limited to individual componentsindependent of other circuit elements, and, for example, wiringresistance in each charge storage node 24 may be used as the resistor R1or R2. In the structure exemplified in FIG. 9, for example, the materialof the connection member 48 x of the unit pixel cell 14 x or thethickness, length, etc. of the plugs therein may differ from thematerial of the connection member 48 y of the unit pixel cell 14 y orthe thickness, length, etc. of the plugs therein. In this case, sincethe resistance value of the connection member 48 x can differ from theresistance value of the connection member 48 y, the connection member 48x and 48 y can be used as the resistors R1 and R2, respectively.

FIG. 13 shows another example of the arrangement of the optical filters.In the structure exemplified in FIG. 13, the unit pixel cell 14 xincludes a color filter 532 (e.g., an R, G, or B filter) and an infraredcut filter 534 facing the color filter 532, and the unit pixel cell 14 yadjacent to the unit pixel cell 14 x includes an IR filter 536. In thisstructure, an operation mode IV described below can be used.

In the operation mode IV, as in the example described with reference toFIG. 12, the bias voltage used when an image is captured is fixed. Thevoltage application circuit 60 applies the second voltage V_(H) to thecounter electrodes 52 x and 52 y. In the state in which the secondvoltage V_(H) is applied, the photoelectric conversion structures 51 xand 51 y in the photoelectric converters 10 each have sensitivity in thevisible range and in the infrared range. In this case, the unit pixelcell 14 x including the color filter 532 and the infrared cut filter 534outputs an image signal for, for example, red, green, or blue light.Therefore, an RGB color image can be formed using the image signal fromthe unit pixel cell 14 x. A pixel value of the unit pixel cell 14 yincluding the IR filter 536 may be complemented, for example, usingpixel values of unit pixel cells therearound. The unit pixel cell 14 yincluding the IR filter 536 outputs an image signal for infrared light.Therefore, an infrared light image can be formed using the image signalfrom the unit pixel cell 14 y. Specifically, with the structureexemplified in FIG. 13, a camera capable of acquiring an RGB color imageand an image using infrared light simultaneously can be obtained.

Table 1 below shows the relation between an operation mode of theimaging device 101 and image signals obtained.

TABLE 1 Operation Combination of optical Switching mode filters in pixelblock voltage Image signals obtained I R filter V_(L) and V_(H) areDuring application of V_(L): G filter applied Red light, green light,and blue G filter alternately light B filter During application ofV_(H): Red light and infrared light, green light and infrared light, andblue light and infrared light II R filter V_(L) Red light, green light,and blue G filter light G filter V_(H) Red light and infrared light, Bfilter green light and infrared light, and blue light and infrared lightIII R filter V_(L) Red light, green light, and blue IR filter light Gfilter V_(H) Infrared light B filter (RGB image signals are discarded)IV R filter and infrared cut filter V_(H) Red light, green light, blueIR filter light, and infrared light G filter and infrared cut filter Bfilter and infrared cut filter

As described above, by using a plurality of optical filters that differin wavelength range in which light can selectively pass through, aplurality of unit pixel cells 14 that have the same photoelectricconversion structure but differ in spectral sensitivity characteristicscan be present in the photosensitive region. Moreover, by changing thebias voltage applied between the pixel electrode 50 and the counterelectrode 52 in each unit pixel cell 14, sensitivity to light in awavelength range corresponding to the absorption spectrum of thematerials forming the second photoelectric conversion layer 512(particularly the second material) can be controlled. Therefore, theimage acquired can be switched, for example, between an image usingvisible light and an image using infrared light according to theswitching voltage supplied from the voltage application circuit 60 tothe unit pixel cells 14. According to the embodiment of the presentdisclosure, an imaging device that can acquire an image using visiblelight and an image using infrared light sequentially or simultaneouslycan be provided. The number of switching voltages is not limited to 2and may be 3 or more.

The operation modes I, II, III, and IV described above are merelyexamples, and various operation modes are applicable to the imagingdevice 101. In the above examples, the sensitivity in the infraredwavelength range is controlled by changing the switching voltage.However, in some combinations of the first and second materials used toform the photoelectric conversion structure 51, the sensitivity in thevisible wavelength range can be controlled by changing the switchingvoltage. For example, an imaging device capable of acquiring an imageusing infrared light during application of the first voltage V_(L) andacquiring a color image during application of the second voltage V_(H)can be obtained. In the operation modes I, II, III, and IV describedabove, the image acquired is switched between an image using visiblelight and an image using infrared light. However, this operation is nota limitation, and the image acquired may be switched between imagesusing light in other wavelength ranges.

A specific value of the switching voltage supplied from the voltageapplication circuit 60 to the unit pixel cells 14 may be setappropriately according to the configuration of the photoelectricconversion structure 51. For example, the voltage application circuit 60may generate, as the switching voltage, a voltage selected within avoltage range in which the sensitivity in the visible range in thephotoelectric conversion structure 51 changes in a specific manner butalmost no change occurs in the sensitivity in the infrared range. Inthis case, sensitivity to a specific wavelength in the visible range canbe increased or reduced. Therefore, an imaging device that can acquireimages with different wavelength distributions in a switchable mannercan be obtained.

EXAMPLES

Samples having the same layered structure as that of the photoelectricconverter 10 described above were produced. For each of the samplesproduced, its external quantum efficiency was measured at differentbiases to evaluate the change in spectral sensitivity characteristicswith respect to the change in bias. The samples were produced asfollows.

Example 1-1

First, a glass substrate was prepared. Next, materials shown in Table 2were sequentially deposited on the glass substrate by vacuum evaporationto thereby form, on the glass substrate, a layered structure including alower electrode, an electron blocking layer, a lower photoelectricconversion layer, an upper photoelectric conversion layer, and an upperelectrode. The thicknesses of the layers formed are also shown in Table2. The lower photoelectric conversion layer was formed by co-evaporationof SnNc and C₇₀. Similarly, the upper photoelectric conversion layer wasformed by co-evaporation of DTDCTB and C₇₀. In the formation of thelower and upper photoelectric conversion layers, the conditions forevaporation were controlled such that the volume ratio of SnNc to C₇₀and the volume ratio of DTDCTB to C₇₀ were 1:1. A sample in Example 1-1was thereby obtained.

TABLE 2 Thickness Layer Material (nm) Upper electrode Al 80 Upperphotoelectric conversion layer DTDCTB:C₇₀ (1:1) 60 Lower photoelectricconversion layer SnNc:C₇₀ (1:1) 60 Electron blocking layer CZBDF 10Lower electrode ITO 150

Next, a spectral sensitivity measurement device CEP-25RR manufactured byBunkoukeiki Co. Ltd. was connected to the lower and upper electrodes,and the external quantum efficiency of the sample in Example 1-1 wasmeasured while the voltage applied between the lower and upperelectrodes was changed. Specifically, with the amount of light suppliedto the measurement target held constant, the external quantum efficiencywas measured while the potential of the lower electrode was changed to−3V, −5V, −8V, −10V, and −10V with the upper electrode grounded. Theapplication of these bias voltages is adapted to the above-describedstructure in which positive charges are collected by the pixel electrode50 in the photoelectric converter 10. Specifically, in this example, thepositive charges generated by photoelectric conversion move toward thelower electrode. The lower and upper electrodes in the sample in Example1-1 correspond to the pixel electrode 50 and the counter electrode 52,respectively, in the photoelectric converter 10 described above.However, since light enters through the glass substrate in themeasurement, ITO is used as the material of the lower electrode, and Alis used as the material of the upper electrode.

FIG. 14 shows the voltage dependence of the external quantum efficiencyof the sample in Example 1-1. The graph shown in FIG. 14 is normalizedsuch that a peak value of the external quantum efficiency is 1. Graphsin figures subsequent to FIG. 14 that show the voltage dependence of theexternal quantum efficiency are also normalized such that a peak valueof the external quantum efficiency is 1.

As can be seen from FIG. 14, when the absolute value of the bias voltageapplied to the lower electrode is small, i.e., the intensity of theelectric field applied between the two electrodes is small, the externalquantum efficiency around the absorption peak of SnNc contained in thelower photoelectric conversion layer is relatively small. Specifically,the sensitivity in the infrared range is low. However, in the visiblerange in which DTDCTB contained in the upper photoelectric conversionlayer exhibits an absorption peak, the external quantum efficiencyobtained is relatively high. As can be seen from FIG. 14, as theabsolute value of the bias voltage applied between the upper and lowerelectrodes is increased, the external quantum efficiency in the infraredrange increases. Specifically, the sensitivity in the wavelength rangecorresponding to the absorption spectrum of SnNc increases with themagnitude of the bias voltage.

For example, the external quantum efficiency at around a wavelength of870 nm corresponding to the absorption peak of SnNc when the potentialof the lower electrode is −11 V is larger by a factor of about 33.7 thanthe external quantum efficiency when the potential of the lowerelectrode is −3 V. Although not shown in FIG. 14, the external quantumefficiency at around a wavelength of 870 nm corresponding to theabsorption peak of SnNc when the potential of the lower electrode is−15V is larger by a factor of about 77.3 than the external quantumefficiency when the potential of the lower electrode is −3 V.

Next, with the sample not irradiated with light, the impedance of theupper photoelectric conversion layer and the impedance of the lowerphotoelectric conversion layer were compared at a prescribed frequency.To measure the impedance, a sample having only the upper photoelectricconversion layer between the lower and upper electrodes and a samplehaving only the lower photoelectric conversion layer between the lowerand upper electrodes were used. The structure of the sample used tomeasure the impedance of the upper photoelectric conversion layer is thesame as the sample in Example 1-1 except that the lower photoelectricconversion layer and the electron blocking layer are not formed and thethickness of the upper photoelectric conversion layer is changed to 200nm. The structure of the sample used to measure the impedance of thelower photoelectric conversion layer is the same as the sample inExample 1-1 except that the upper photoelectric conversion layer and theelectron blocking layer are not formed and the thickness of the lowerphotoelectric conversion layer is changed to 200 nm. To measure andanalyze the impedance, ModuLab XM ECS manufactured by TOYO Corporationand Zplot software were used. A frequency sweep mode was used as theoperation mode. The amplitude was set to 10 mV, and the frequency waschanged from 1 Hz to 1 MHz. In the measurement, a start delay of 5second was used. As for the upper and lower photoelectric conversionlayers, their impedance values at a bias voltage between the upper andlower electrodes of −8 V and a frequency of 1 Hz with the upper andlower photoelectric conversion layers not irradiated with light werecompared.

The impedance of the upper photoelectric conversion layer containingDTDCTB at a bias voltage of −8 V and a frequency of 1 Hz was 7.5×10⁶Ω),and the impedance of the lower photoelectric conversion layer containingSnNc was 4.2×10³Ω). The impedance of the upper photoelectric conversionlayer is larger by a factor of about 1,800 than the impedance of thelower photoelectric conversion layer.

FIG. 15 shows the relation between the applied electric field and theexternal quantum efficiency of the sample in Example 1-1 at wavelengthsof 460 nm, 540 nm, 680 nm, and 880 nm. The horizontal axis of the graphshown in FIG. 15 is a value obtained by dividing the bias voltageapplied between the upper and lower electrodes by the total thickness ofthe upper photoelectric conversion layer, the lower photoelectricconversion layer, and the electron blocking layer. Specifically, thehorizontal axis of the graph in FIG. 15 corresponds to the magnitude ofthe electric field applied between the upper and lower electrodes.

As can be seen from the example shown in FIG. 15, the external quantumefficiency for light with a wavelength of 880 nm is almost zero at anelectric field intensity of about less than 4×10⁵ V/cm and startsincreasing at an electric field intensity of about 4×10⁵ V/cm or more.When a sufficiently large bias is applied to the photoelectricconversion structure including the layered structure including the firstand second photoelectric conversion layers (see, for example, FIG. 3), asufficiently large bias can be applied to a layer having a smallerimpedance among the two photoelectric conversion layers. As can be seenfrom FIG. 15, by applying a sufficiently large bias to the layer havinga smaller impedance among the two photoelectric conversion layers, theexternal quantum efficiency of this layer exhibits a relatively highexternal quantum efficiency.

As can be seen from FIG. 15, the external quantum efficiencies atwavelengths of 460 nm, 540 nm, 680 nm, and 880 nm tend to saturate whenthe magnitude of the electric field between the upper and lowerelectrodes is about 9×10⁵ V/cm or higher. Specific values of the firstvoltage VA and the second voltage VB can be determined, for example, asfollows. The second voltage VB used may be a voltage at which theintensity of the electric field applied to the photoelectric conversionstructure is 70% or more of the intensity of the electric field at whichthe external quantum efficiency for light in a first wavelength range(e.g., the visible range) and the external quantum efficiency for lightin a second wavelength range (e.g., the infrared range) are saturated.The first voltage VA used may be a voltage at which the intensity of theelectric field applied to the photoelectric conversion structure 51 is30% or less of the intensity of the electric field at which the externalquantum efficiency for light in the first wavelength range is saturated.The state in which the external quantum efficiency is about 0.2 or moremay be defined as a sensitive state. The first voltage VA and the secondvoltage VB may be appropriately determined in consideration of thethicknesses etc. of the first photoelectric conversion layer 511 and thesecond photoelectric conversion layer 512. According to the results ofstudies by the inventors, the imaging device is practically useful whenthe external quantum efficiency of the photoelectric conversionstructure under application of the second voltage VB at a wavelengthcorresponding to the absorption peak of the second material included inthe second photoelectric conversion layer 512 is about twice or more theexternal quantum efficiency under application of the first voltage VA,but this depends on the use of the imaging device. Alternatively, theexternal quantum efficiency of the photoelectric conversion structureunder application of the second voltage VB at a wavelength correspondingto the absorption peak of the first material included in the firstphotoelectric conversion layer 511 is about twice or more the externalquantum efficiency under application of the first voltage VA.

Example 1-2

A sample in Example 1-2 was produced in substantially the same manner asfor the sample in Example 1-1 except that a mixture layer containingSnNc and DTDCTB was disposed between the lower and upper photoelectricconversion layers. Table 3 below shows the materials and thicknesses ofthe layers in the sample in Example 1-2. The mixture layer was formed byco-evaporation of three materials, i.e., SnNc, DTDCTB, and C₇₀. In theformation of the mixture layer, the evaporation conditions werecontrolled such that the volume ratio of SnNc, DTDCTB, and C₇₀ was1:1:8. In the formation of the lower photoelectric conversion layer, theevaporation conditions were controlled such that the volume ratio ofSnNc to C₇₀ was 1:4. In the formation of the upper photoelectricconversion layer, the evaporation conditions were controlled such thatthe volume ratio of DTDCTB to C₇₀ was 1:4.

TABLE 3 Thickness Layer Material (nm) Upper electrode Al 80 Upperphotoelectric conversion layer DTDCTB:C₇₀ (1:4) 50 Mixture layerSnNc:DTDCTB:C₇₀ 20 (1:1:8) Lower photoelectric conversion layer SnNc:C₇₀(1:4) 50 Electron blocking layer CZBDF 10 Lower electrode ITO 150

The voltage dependence of the external quantum efficiency of the samplein Example 1-2 was measured in the same manner as for the sample inExample 1-1. FIG. 16 shows the voltage dependence of the externalquantum efficiency of the sample in Example 1-2.

As shown in FIG. 16, in the sample in Example 1-2, as in the sample inExample 1-1, the external quantum efficiency at around a wavelength of870 nm corresponding to the absorption peak of SnNc contained in thelower photoelectric conversion layer increases as the absolute value ofthe bias voltage applied to the lower electrode increases. As can beseen from FIG. 16, even in the structure in which the mixture layercontaining both the first and second materials is disposed between thefirst and second photoelectric conversion layers, the effect of changingthe sensitivity by changing the bias voltage can be obtained.

Example 1-3

A sample in Example 1-3 was produced in substantially the same manner asfor the sample in Example 1-1 except that ClAlPc and C₇₀ were used asthe materials for forming the lower photoelectric conversion layer. Inthe formation of the lower photoelectric conversion layer, theevaporation conditions were controlled such that the volume ratio ofClAlPc to C₇₀ was 1:9. Table 4 below shows the materials and thicknessesof the layers in the sample in Example 1-3.

TABLE 4 Thickness Layer Material (nm) Upper electrode Al 80 Upperphotoelectric conversion layer DTDCTB:C₇₀ (1:9) 60 Lower photoelectricconversion layer ClAlPc:C₇₀ (1:1) 60 Electron blocking layer CZBDF 10Lower electrode ITO 150

The voltage dependence of the external quantum efficiency of the samplein Example 1-3 was measured in the same manner as for the sample inExample 1-1. FIG. 17 shows the voltage dependence of the externalquantum efficiency of the sample in Example 1-3.

As shown in FIG. 17, in the sample in Example 1-3, as the magnitude ofthe electric field applied between the two electrodes increases, theexternal quantum efficiency in the infrared range increases.Specifically, in the sample in Example 1-3, as the absolute value of thebias voltage applied between the upper and lower electrodes increases,the external quantum efficiency at around a wavelength of 750 nmcorresponding to the absorption peak of ClAlPc contained in the lowerphotoelectric conversion layer increases. In other words, thesensitivity in the infrared range is changed by changing the biasvoltage. For example, the external quantum efficiency at the wavelengthcorresponding to the absorption peak of ClAlPc when the potential of thelower electrode is −5 V is larger by a factor of about 6.55 than theexternal quantum efficiency when the potential of the lower electrode is−1 V.

Next, the same sample as the sample in Example 1-3 except that only theupper photoelectric conversion layer was disposed between the lower andupper electrodes and the same sample as the sample in Example 1-3 exceptthat only the lower photoelectric conversion layer was disposed betweenthe lower and upper electrodes were produced in the same manner as inExample 1-1, and the impedance of the upper photoelectric conversionlayer and the impedance of the lower photoelectric conversion layer weremeasured. The thickness of the upper photoelectric conversion layer andthe thickness of the lower photoelectric conversion layer in themeasurement samples were 200 nm. Table 5 below shows the results of theimpedance measurement. The impedance values below are values when thebias voltage applied between the lower and upper electrodes is −8V andthe frequency is 1 Hz with the samples not irradiated with light.

TABLE 5 Sample Layer Donor-acceptor ratio Impedance (Ω) Example 1-3Upper photoelectric DTDCTB:C₇₀ 1.2 × 10⁷ conversion layer (1:9) Lowerphotoelectric ClAlPc:C₇₀ 6.3 × 10⁴ conversion layer (1:1)

As can be seen from Table 5, in the sample in Example 1-3, the impedanceof the upper photoelectric conversion layer is larger by a factor ofabout 190 than the impedance of the lower photoelectric conversionlayer.

The ionization potential of DTDCTB used to form the upper photoelectricconversion layer in each of the samples in Examples 1-1 and 1-3is about5.6 eV. The ionization potential of SnNc used to form the lowerphotoelectric conversion layer in the sample in Example 1-1 is 5.0 eV,and the ionization potential of ClAlPc used to form the lowerphotoelectric conversion layer in the sample in Example 1-3 is 5.5 eV.Therefore, in the samples in Examples 1-1 and 1-3, no potential barrierfor holes is formed between the lower and upper photoelectric conversionlayers. This shows that, even when there is no potential barrier forholes, the sensitivity can be changed by changing the bias voltage whena difference in impedance is present between the two photoelectricconversion layers in the layered structure. The impedance of the upperphotoelectric conversion layer and the impedance of the lowerphotoelectric conversion layer may be impedances at a frequency of 1 Hzwith the upper and lower photoelectric conversion layers not irradiatedwith light.

Example 2-1

A sample in Example 2-1 was produced in basically the same manner as inExample 1-1 except that SnNc and C₇₀ were used as the materials formingthe upper photoelectric conversion layer and rubrene and C₇₀ were usedas the materials forming the lower photoelectric conversion layer. Thevolume ratio of SnNc to C₇₀ and the volume ratio of rubrene to C₇₀ werecontrolled to 1:4. Table 6 below shows the materials and thicknesses ofthe layers in the sample in Example 2-1. As shown in Table 6, thethickness of the upper photoelectric conversion layer and the thicknessof the lower photoelectric conversion layer were 200 nm.

TABLE 6 Layer Material Thickness (nm) Upper electrode Al 80 Upperphotoelectric conversion layer SnNc:C₇₀ (1:4) 200 Lower photoelectricconversion layer Rubrene:C₇₀ (1:4) 200 Electron blocking layer CZBDF 10Lower electrode ITO 150

Comparative Example 1

A sample in Comparative Example 1 was produced in the same manner as inExample 2-1 except that rubrene and C₇₀ were used as the materialsforming the upper photoelectric conversion layer and SnNc and C₇₀ wereused as the materials forming the lower photoelectric conversion layer.Specifically, the sample in Comparative Example 1 has a structure inwhich the upper and lower photoelectric conversion layers in the samplein Example 2-1 are exchanged with each other. Table 7 below shows thematerials and thicknesses of the layers in the sample in ComparativeExample 1.

TABLE 7 Layer Material Thickness (nm) Upper electrode Al 80 Upperphotoelectric conversion layer Rubrene:C₇₀ (1:4) 200 Lower photoelectricconversion layer SnNc:C₇₀ (1:4) 200 Electron blocking layer CZBDF 10Lower electrode ITO 150

The voltage dependence of the external quantum efficiency of each of thesamples in Example 2-1 and Comparative Example 1 was measured in thesame manner as for the sample in Example 1-1. FIG. 18 shows the voltagedependence of the external quantum efficiency of the sample in Example2-1, and FIG. 19 shows the voltage dependence of the external quantumefficiency in the sample in Comparative Example 1.

As shown by a broken line circle in FIG. 18, in the sample in Example2-1, as the intensity of the electric field applied between the twoelectrodes increases, the external quantum efficiency in the infraredrange increases. In this example, when the bias voltage applied betweenthe upper and lower electrodes is smaller than about −5 V, sufficientsensitivity is achieved in the infrared range. Specifically, in thesample in Example 2-1, as the absolute value of the bias voltage appliedbetween the upper and lower electrodes increases, the external quantumefficiency around the absorption peak of SnNc contained in the lowerphotoelectric conversion layer increases. For example, the externalquantum efficiency at around a wavelength of 870 nm corresponding to theabsorption peak of SnNc when the potential of the lower electrode is −10V is larger by a factor of 4.27 than the external quantum efficiencywhen the potential of the lower electrode is −3 V.

However, as shown in FIG. 19, in the sample in Comparative Example 1, asthe intensity of the electric field applied between the two electrodesincreases, both the external quantum efficiency in the infrared rangeand the external quantum efficiency in the visible range increase.Specifically, in the sample in Comparative Example 1, the sensitivity inthe infrared range does not change in a specific manner when the biasvoltage is changed.

Next, the same sample as the sample in Example 2-1 except that only theupper photoelectric conversion layer was disposed between the lower andupper electrodes and the same sample as the sample in Example 2-1 exceptthat only the lower photoelectric conversion layer was disposed betweenthe lower and upper electrodes were produced in the same manner as inExample 1-1. Moreover, the same sample as the sample in ComparativeExample 1 except that only the upper photoelectric conversion layer wasdisposed between the lower and upper electrodes and the same sample asthe sample in Comparative Example 1 except that only the lowerphotoelectric conversion layer was disposed between the lower and upperelectrodes were produced in the same manner as in Example 1-1. For eachof the samples, the impedance of the upper photoelectric conversionlayer or the impedance of the lower photoelectric conversion layer wasmeasured at a prescribed frequency with the sample not irradiated withlight. The thickness of the upper photoelectric conversion layer or thelower photoelectric conversion layer in each measurement sample was 200nm. Table 8 below shows the results of the impedance measurement.

TABLE 8 Sample Layer Donor-acceptor ratio Impedance (Ω) Example 2-1Upper photoelectric SnNc:C₇₀ 1.0 × 10⁴ conversion layer (1:4) Lowerphotoelectric Rubrene:C₇₀ 9.0 × 10³ conversion layer (1:4) ComparativeUpper photoelectric Rubrene:C₇₀ 9.0 × 10³ Example 1 conversion layer(1:4) Lower photoelectric SnNc:C₇₀ 1.0 × 10⁴ conversion layer (1:4)

As can be seen from Table 8, in the sample in Comparative Example 1, theimpedance of the upper photoelectric conversion layer is lower than theimpedance of the lower photoelectric conversion layer. In the sample inExample 2-1, the impedance of the upper photoelectric conversion layeris larger than the impedance of the lower photoelectric conversionlayer. However, the ratio of the impedance of the upper photoelectricconversion layer to the impedance of the lower photoelectric conversionlayer is about 1.1, and the difference in impedance between the lowerand upper photoelectric conversion layers is not large.

Next, attention is focused on the ionization potentials of rubrene andSnNc. The ionization potential of rubrene is 5.35 eV, and the ionizationpotential of SnNc is 5.0 eV. Therefore, in the sample in Example 2-1, apotential barrier of 0.35 eV for positive charges moving toward thelower electrode is present between the HOMO level of rubrene and theHOMO level of SnNc (see FIG. 6). In the sample in Comparative Example 1,no potential barrier for positive charges moving toward the lowerelectrode is present between the HOMO level of rubrene and the HOMOlevel of SnNc (see FIG. 8). The reason that the sensitivity in theinfrared range does not change in a specific manner in the sample inComparative Example 1 but changes in a specific manner in the sample inExample 2-1 may be that the potential barrier for holes is formedbetween the two photoelectric conversion layers in the sample in Example2-1.

Example 2-2

Materials shown in Table 9 below were sequentially deposited on a glasssubstrate by vacuum evaporation to thereby produce a sample in Example2-2. The lower photoelectric conversion layer was formed byco-evaporation of ClAlPc and C₆₀, and the upper photoelectric conversionlayer was formed by co-evaporation of α-6T and C₇₀. When the lowerphotoelectric conversion layer was formed, the evaporation conditionswere controlled such that the volume ratio of ClAlPc to C₆₀ was 1:4.When the upper photoelectric conversion layer was formed, theevaporation conditions were controlled such that the volume ratio ofα-6T to C₇₀ was 1:1.

TABLE 9 Layer Material Thickness (nm) Upper electrode Al 80 Upperphotoelectric conversion layer α-6T:C₇₀ (1:1) 60 Lower photoelectricconversion layer ClAlPc:C₆₀ (1:4) 60 Electron blocking layer CZBDF 10Lower electrode ITO 150

FIG. 20 is an energy diagram of the sample in Example 2-2. As shown inFIG. 20, the ionization potential of ClAlPc is 5.5 eV, and theionization potential of α-6T is 5.3 eV. In the sample in Example 2-2, apotential barrier of 0.2 eV is formed between the HOMO level of ClAlPcand the HOMO level of α-6T.

The voltage dependence of the external quantum efficiency of the samplein Example 2-2 was measured in the same manner as for the sample inExample 1-1. FIG. 21 shows the voltage dependence of the externalquantum efficiency of the sample in Example 2-2. As shown in FIG. 21, inthe sample in Example 2-2, as the absolute value of the bias voltageapplied to the lower electrode increases, the external quantumefficiency at around a wavelength of 440 nm corresponding to theabsorption peak of α-6T increases. In other words, the external quantumefficiency in the visible range increases. Specifically, in thisexample, the effect of changing the sensitivity in the visible range bychanging the bias voltage can be obtained.

Comparative Example 2

A sample in Comparative Example 2 was produced in the same manner as inExample 2-2 except that the materials for forming the upperphotoelectric conversion layer and the materials for forming the lowerphotoelectric conversion layer were exchanged with each other. Table 10below shows the materials and thicknesses of the layers in ComparativeExample 2.

TABLE 10 Layer Material Thickness (nm) Upper electrode Al 80 Upperphotoelectric conversion layer ClAlPc:C₆₀ (1:4) 60 Lower photoelectricconversion layer α-6T:C₇₀ (1:1) 60 Electron blocking layer CZBDF 10Lower electrode ITO 150

FIG. 22 is an energy diagram of the sample in Comparative Example 2. Ascan be seen from FIG. 22, in this example, no potential barrier forholes is formed between the HOMO level of ClAlPc and the HOMO level ofα-6T.

The voltage dependence of the external quantum efficiency of the samplein Comparative Example 2 was measured in the same manner as for thesample in Example 1-1. FIG. 23 shows the voltage dependence of theexternal quantum efficiency of the sample in Comparative Example 2. Asshown in FIG. 23, in the sample in Comparative Example 2, even when thebias voltage applied to the lower electrode is changed, no significantchange is found in the graph of the external quantum efficiency, so thatthe sensitivity is not changed by changing the bias voltage.

As can be seen from FIGS. 18 to 23, by forming a potential barrier forpositive charges between the HOMO level of a material included in theupper photoelectric conversion layer and the HOMO level of a materialincluded in the lower photoelectric conversion layer, the sensitivitycan be changed by changing the bias voltage. As can be seen fromcomparison between Example 2-2 and Comparative Example 2, byappropriately selecting the materials in the two photoelectricconversion layers in the layered structure, the external quantumefficiency can be increased in a specific manner also in the visiblerange.

As can be seen from comparison between Example 2-2 and ComparativeExample 2, when, in the layered structure including the twophotoelectric conversion layers in the photoelectric conversionstructure, one of the two photoelectric conversion layers that is closerto a lower potential electrode (the lower electrode in this example)contains a material having an ionization potential larger by about atleast 0.2 eV than the ionization potential of a material of the otherphotoelectric conversion layer, the effect of increasing the externalquantum efficiency in a specific manner can be obtained not only in theinfrared range but also in a specific wavelength range. For example, theionization potential of Si(OSiR₃)₂Nc is 5.4 eV, and the ionizationpotential of CuPc is 5.2 eV. Therefore, when Si(OSiR₃)₂Nc is used as thefirst material and CuPc is used as the second material, it is expectedthat the sensitivity in the visible range can be changed in a specificmanner. CuPc may be used instead of rubrene in Example 2-2.

As described above, the embodiment of the present disclosure can providean imaging device in which the spectral sensitivity characteristics canbe electrically changed. In the embodiment of the present disclosure,the external quantum efficiency in a specific wavelength range can beselectively increased using the bias voltage applied to thephotoelectric conversion structure of the photoelectric converter. Forexample, the bias voltage can be supplied from the voltage applicationcircuit disposed outside the photosensitive region to the photoelectricconverter of each pixel cell. By using the voltage application circuitthat can generate at least two voltage levels in a switchable manner,one selected from the plurality of bias voltages can be selectivelyapplied to the photoelectric converter according to the polarity of thecharges collected by the pixel electrode and the specific layeredstructure in the photoelectric conversion structure. For example, byselecting the bias voltage to be applied to the photoelectric converterfrom the plurality of bias voltages, the image acquirable wavelengthband can be changed. Specifically, with the imaging device in theembodiment of the present disclosure, an image using light in awavelength range (e.g., visible light) and an image using light inanother wavelength range (e.g., infrared light), for example, can beobtained.

The voltage application circuit 60 in the above-described embodiment isconfigured such that the switching voltage can be applied independentlyto each of the rows of unit pixel cells 14 arranged two-dimensionally.However, the voltage application circuit 60 may be configured such thatthe same switching voltage is applied independently to each two rows ofunit pixel cells 14 or to all the unit pixel cells 14 in thephotosensitive region. Alternatively, the voltage application circuit 60may be configured such that different voltages can be applied todifferent unit pixel cells 14 or that different voltages can be appliedto different groups other than rows and columns, e.g., different groupsof adjacent unit pixel cells 14.

In one example described in the above embodiment, the transistors ineach unit pixel cell such as the amplification transistor 11, the resettransistor 12, and the address transistor 13 are N-channel MOSFETs.However, the transistors in the embodiment of the present disclosure arenot limited to the N-channel MOSFETs. The transistors in each unit pixelcell may be N-channel MOSFETs and may be P-channel MOSFETs. It isunnecessary that the transistors include only N-channel MOSFETs or onlyP-channel MOSFETs. In addition to the FETs, bipolar transistors may beused as the transistors in each unit pixel cell.

What is claimed is:
 1. An imaging device comprising at least one unitpixel cell including a photoelectric converter that converts incidentlight into electric charges, wherein the photoelectric converterincludes: a first electrode; a second electrode configured to transmitthe incident light; a first photoelectric conversion layer disposedbetween the first electrode and the second electrode and containing afirst material having an absorption peak at a first wavelength; and asecond photoelectric conversion layer disposed between the firstphotoelectric conversion layer and the second electrode and containing asecond material having an absorption peak at a second wavelengthdifferent from the first wavelength, and an absolute value of anionization potential of the first material is larger by at least 0.2 eVthan an absolute value of an ionization potential of the secondmaterial.
 2. The imaging device according to claim 1, wherein one of thefirst and second wavelengths falls within a visible wavelength range,and the other of the first and second wavelengths falls within aninfrared wavelength range.
 3. The imaging device according to claim 1,wherein the first wavelength falls within a visible wavelength range,and the second wavelength falls within an infrared wavelength range. 4.The imaging device according to claim 1, wherein the first materialcontains electron-donating molecules, and the second material containselectron-donating molecules.
 5. The imaging device according to claim 1,wherein the first photoelectric conversion layer further containselectron-accepting molecules, and the second photoelectric conversionlayer further contains electron-accepting molecules.
 6. The imagingdevice according to claim 1, further comprising a voltage applicationcircuit electrically connected to the second electrode, wherein thevoltage application circuit selectively applies a first voltage or asecond voltage different from the first voltage between the firstelectrode and the second electrode.
 7. The imaging device according toclaim 6, wherein an absolute value of the second voltage is larger thanan absolute value of the first voltage, an external quantum efficiencyof the photoelectric converter at the second wavelength when the secondvoltage is applied between the first electrode and the second electrodeis larger than an external quantum efficiency of the photoelectricconverter at the second wavelength when the first voltage is appliedbetween the first electrode and the second electrode, and a differencebetween the external quantum efficiency of the photoelectric converterat the second wavelength when the second voltage is applied and theexternal quantum efficiency of the photoelectric converter at the secondwavelength when the first voltage is applied is larger than a differencebetween an external quantum efficiency of the photoelectric converter atthe first wavelength when the second voltage is applied and an externalquantum efficiency of the photoelectric converter at the firstwavelength when the first voltage is applied.
 8. The imaging deviceaccording to claim 7, wherein the external quantum efficiency of thephotoelectric converter at the second wavelength when the second voltageis applied is at least twice the external quantum efficiency of thephotoelectric converter at the second wavelength when the first voltageis applied.
 9. The imaging device according to claim 1, wherein thephotoelectric converter further includes a mixture layer containing thefirst material and the second material.
 10. The imaging device accordingto claim 1, wherein the at least one unit pixel cell comprises a firstunit pixel cell and a second unit pixel cell.
 11. The imaging deviceaccording to claim 10, further comprising a color filter facing thesecond electrode of the first unit pixel cell.
 12. The imaging deviceaccording to claim 11, further comprising an infrared pass filter facingthe second electrode of the second unit pixel cell.
 13. The imagingdevice according to claim 12, further comprising an infrared cut filterfacing the color filter.
 14. The imaging device according to claim 10,wherein the second electrode of the first unit pixel cell and the secondelectrode of the second unit pixel cell are a single continuouselectrode.
 15. The imaging device according to claim 10, wherein thefirst photoelectric conversion layer of the first unit pixel cell andthe first photoelectric conversion layer of the second unit pixel cellare a single continuous layer, and the second photoelectric conversionlayer of the first unit pixel cell and the second photoelectricconversion layer of the second unit pixel cell are a single continuouslayer.
 16. The imaging device according to claim 1, wherein the firstwavelength falls within an infrared wavelength range, and the secondwavelength falls within a visible wavelength range.
 17. The imagingdevice according to claim 1, wherein one of the first photoelectricconversion layer and the second photoelectric conversion layer has anabsorption peak at 800 nm or more.
 18. The imaging device according toclaim 1, wherein an absorption peak of one of the first photoelectricconversion layer and the second photoelectric conversion layer extendsto 900 nm or more.
 19. The imaging device according to claim 1, whereinan absorption peak of one of the first photoelectric conversion layerand the second photoelectric conversion layer extends to 1000 nm ormore.
 20. The imaging device according to claim 6, wherein a ratio ofchange of an external quantum efficiency of the photoelectric converterat the second wavelength when a voltage between the first electrode andthe second electrode is switched between the first voltage and thesecond voltage is larger than a ratio of change of an external quantumefficiency of the photoelectric converter at the first wavelength whenthe voltage is switched between the first voltage and the secondvoltage.
 21. The imaging device according to claim 6, wherein anexternal quantum efficiency of the photoelectric converter changes in awavelength region of 800 nm or more when a voltage between the firstelectrode and the second electrode is switched between the first voltageand the second voltage.
 22. The imaging device according to claim 6,wherein an external quantum efficiency of the photoelectric converterchanges in a wavelength region of 900 nm or more when a voltage betweenthe first electrode and the second electrode is switched between thefirst voltage and the second voltage.